Aqueous redox flow batteries featuring improved cell design characteristics

ABSTRACT

Provided are compositions having the formula M n Ti(L1)(L2)(L3) wherein L1 is a catecholate, and L2 and L3 are each independently selected from catecholates, ascorbate, citrate, glycolates, a polyol, gluconate, glycinate, hydroxyalkanoates, acetate, formate, benzoates, malate, maleate, phthalates, sarcosinate, salicylate, oxalate, a urea, polyamine, aminophenolates, acetylacetone or lactate; each M is independently Na, Li, or K; n is 0 or an integer from 1-6. Also provided are energy storage systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/164,839, filed on Jan. 27, 2014, which is a continuation-in-part ofU.S. patent application Ser. No. 13/796,004, filed on Mar. 12, 2013 andnow U.S. Pat. No. 8,691,413, and a continuation-in-part of U.S. patentapplication Ser. No. 13/795,878, filed on Mar. 12, 2013 and now U.S.Pat. No. 8,753,761, each of which is incorporated herein by reference inits entirety. U.S. patent application Ser. No. 13/796,004, in turn,claims the benefit of priority of U.S. Provisional Patent Application61/739,538, filed on Dec. 19, 2012, U.S. Provisional Patent Application61/739,140, filed on Dec. 19, 2012, U.S. Provisional Patent Application61/738,546, filed on Dec. 18, 2012, U.S. Provisional Patent Application61/683,260, filed on Aug. 15, 2012, and U.S. Provisional PatentApplication 61/676,473, filed on Jul. 27, 2012. U.S. patent applicationSer. No. 13/795,878, in turn, claims the benefit of priority of U.S.Provisional Patent Application 61/739,145, filed on Dec. 19, 2012, U.S.Provisional Patent Application 61/738,546, filed on Dec. 18, 2012, U.S.Provisional Patent Application 61/683,260, filed on Aug. 15, 2012, andU.S. Provisional Patent Application 61/676,473, filed on Jul. 27, 2012.Each of the foregoing applications is incorporated herein by referencein its entirety.

TECHNICAL FIELD

This disclosure relates to the field of energy storage systems,including electrochemical energy storage systems, batteries, and flowbattery systems and methods of operating the same.

BACKGROUND

There exists a long-felt need for safe, inexpensive, easy-to-use, andreliable technologies for energy storage. Large scale energy storageenables diversification of energy supply and optimization of the energygrid, including increased penetration and utilization of renewableenergies. Existing renewable-energy systems (e.g., solar- and wind-basedsystems) enjoy increasing prominence as energy producers explorenon-fossil fuel energy sources, however storage is required to ensure ahigh quality energy supply when sunlight is not available and when winddoes not blow.

Electrochemical energy storage systems have been proposed forlarge-scale energy storage. To be effective, these systems must be safe,reliable, low-cost, and highly efficient at storing and producingelectrical power. Flow batteries, compared to other electrochemicalenergy storage devices, offer an advantage for large-scale energystorage applications owing to their unique ability to decouple thefunctions of power density and energy density. Flow batteries aregenerally comprised of negative and positive active materialelectrolytes, which are flowed separately across either side of amembrane or separator in an electrochemical cell. The battery is chargedor discharged through electrochemical reactions of the active materialsinside the electrochemical cell.

Existing flow batteries have suffered from the reliance on batterychemistries and cell designs that result in either high cell resistanceor active materials crossing over the membrane and mixing. Thisphenomenon results in low cell and system performance (e.g. round tripenergy efficiency) and poor cycle life, among others. To be effective,the flow battery chemistry and cell components must be chosen andoptimized to afford low cell resistance and low active materialcrossover. Despite significant development effort, no flow batterytechnology has yet achieved this combination. Accordingly, there is aneed in the art for improved flow battery chemistry and cell designcharacteristics.

SUMMARY

The present disclosure addresses these challenges by providingcompositions that are useful, inter alia, as active materials in flowbatteries. The present disclosure provides, inter alia, compositionshaving the formula:

M_(n)Ti(L1)(L2)(L3)

wherein:L1 is a catecholate, andL2 and L3 are each independently selected from catecholates, ascorbate,citrate, glycolates, a polyol, gluconate, glycinate, hydroxyalkanoates,acetate, formate, benzoates, malate, maleate, phthalates, sarcosinate,salicylate, oxalate, a urea, polyamine, aminophenolates, acetylacetoneor lactate;each M is independently Na, Li, or K;n is 0 or an integer from 1-6; andprovided that when both L1 and L2 are a catecholate, L3 is not oxalate,urea, catecholate or acetylacetone.

In some embodiments, the catecholate comprises 1,2-dihydroxybenzene,1,2,3-trihydroxybenzene, 1,2,4-trihydroxybenzene or a mixture thereof.Preferred embodiments include compositions having the formula

M_(n)Ti(catecholate)₂(hydroxycatecholate) or M_(n)Ti(catecholate)₃.

Another aspect of the invention concerns an aqueous solution, comprisingone or more compositions having the formula

M_(n)Ti(L1)(L2)(L3)

wherein:L1 is a catecholate, andL2 and L3 are each independently selected from catecholates, ascorbate,citrate, glycolates, a polyol, gluconate, glycinate, hydroxyalkanoates,acetate, formate, benzoates, malate, maleate, phthalates, sarcosinate,salicylate, oxalate, a urea, polyamine, aminophenolates, acetylacetoneor lactate;each M is independently Na, Li, or K;n is 0 or an integer from 1-6.

Some solutions additionally comprise a buffer. Suitable buffers include,but are not limited to a salt of phosphate, borate, carbonate, silicate,tris(hydroxymethyl)aminomethane (tris),4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES),piperazine-N,N′-bis(ethanesulfonic acid) (PIPES), and combinationsthereof.

In some embodiments, the aqueous solution has a pH of 1-13, 2-12, 4-10or 6-8.

In some aspects, the disclosure provides flow batteries comprising oneor more of the compositions disclosed herein. In some embodiments, thecomposition is an active material.

BRIEF DESCRIPTION OF THE DRAWINGS

The present application is further understood when read in conjunctionwith the appended drawings. For the purpose of illustrating the subjectmatter, there are shown in the drawings exemplary embodiments of thesubject matter; however, the presently disclosed subject matter is notlimited to the specific methods, devices, and systems disclosed. Inaddition, the drawings are not necessarily drawn to scale. In thedrawings:

FIG. 1 depicts a schematic of an exemplary flow battery.

FIG. 2 provides exemplary stability performance data obtained during 250charge/discharge cycles for a 5 cm² system based on Ti^(4+/3+)(cat)₃^(2−/3−) and Fe^(3+/2+)(CN)₆ ^(3−/4−), as described in Example 2.

FIG. 3 provides a charge/discharge trace for an exemplary flow batteryaccording to the present invention as described in Example 2. Thisexample contains Ti^(4+/3+)(cat)₃ ^(2−/3−) and Fe^(3+/2+)(CN)₆ ^(3−/4−)as first and second electrolytes, respectively. The battery was chargedfrom 0% SOC to 60% SOC and then discharged to 40% SOC at a currentdensity of 200 mA/cm² and a RT Voltage efficiency of ˜76%.

FIG. 4 provides current efficiency data obtained for a system based onTi^(4+/3+)(cat)₃ ^(2−/3−) and Fe^(3+/2+)(CN)₆ ^(3−/4−), as described inExample 3.

FIG. 5 provides voltage efficiency data, as a function of currentdensity, for a system based on Ti^(4+/3+)(cat)₂(pyrogallate)^(2−/3−) andFe^(3+/2+)(CN)₆ ^(3−/4−), as described in Example 4.

FIG. 6 provides voltage efficiency data, as a function of currentdensity, for a system based on Ti^(4+/3+)(cat)₃ ^(2−/3−) andFe^(3+/2+)(CN)₆ ^(3−/4−), as described in Example 4.

FIG. 7 provides a charge/discharge trace for a flow battery of thepresent invention. This example contains Fe^(3+/2+)(cat)₃ ^(3−/4−) andFe^(3+/2+)(CN)₆ ^(3−/4−) as first and second electrolytes, respectively.The battery was charged from 0% SOC to 60% SOC and then discharged to40% SOC at a current density of 100 mA/cm² and a RT voltage efficiencyof ca. 82%.

FIG. 8 provides cyclic voltammogram, CV traces for Al(cit)²(cat)^(2−/3−)in pH 11.5 Na₂SO₄ electrolyte recorded at a glassy carbon electrode.

FIG. 9 provides CV traces for titanium tris-pyrogallate over a range ofoperating potentials. The data were generated using solutions of 75 mMNaK[Ti(pyrogallate)₃] at a pH of 9.8 and 1 M Na₂SO₄, recorded at aglassy carbon electrode.

FIG. 10 provides CV traces for iron tris-catecholate over a range ofoperating potentials. The data were generated using solutions of 1MNaK[Fe(catecholate)₃] at a pH of 11, and 3 M Na/KCl, recorded at aglassy carbon electrode.

FIG. 11 provides a CV trace for titanium bis-catecholatemono-pyrogallate over a range of operating potentials. The data weregenerated using solutions of 1.6 M NaK[Ti(catecholate)₂(pyrogallate)] ata pH of 11, recorded at a glassy carbon electrode.

FIG. 12 provides a CV trace for titanium bis-catecholate monolactateover a range of operating potentials. The data were generated usingsolutions of 0.75 M NaK[Ti(catecholate)₂(lactate)] at a pH of 9,recorded at a glassy carbon electrode.

FIG. 13 provides a CV trace for titanium bis-catecholate mono-gluconateover a range of operating potentials. The data were generated usingsolutions of 1.5 M NaK[Ti(catecholate)₂(gluconate)] at a pH of 9,recorded at a glassy carbon electrode.

FIG. 14 provides a CV trace for titanium bis-catecholate mono-ascorbateover a range of operating potentials. The data were generated usingsolutions of 1.5 M NaK[Ti(catecholate)₂(ascorbate)] at a pH of 10,recorded at a glassy carbon electrode.

FIG. 15 provides a CV trace for titanium tris-catecholate over a rangeof operating potentials. The data were generated using solutions of 1.5M Na₂[Ti(catecholate)₃] at a pH of 11, recorded at a glassy carbonelectrode.

FIG. 16 provides a CV trace for titanium mono-catecholatemono-pyrogallate mono-lactate over a range of operating potentials. Thedata were generated using solutions of 1.5 MNaK[Ti(catecholate)(pyrogallate)(lactate)] at a pH of 8.5, recorded at aglassy carbon electrode.

FIG. 17 provides a CV trace for titanium tris-citrate over a range ofoperating potentials. The data were generated using solutions of 0.5 MNa₄[Ti(citrate)₃] at a pH of 5, recorded at a glassy carbon electrode.

FIG. 18 provides a CV trace from a solution of 1.5 M [Fe(CN)₆]⁴⁻obtained at a glassy carbon disk working electrode at several scan ratesusing 0.1 M sodium potassium hydrogen phosphate as the supportingelectrolyte, as described in Example 5.11. The ratio of Na⁺/K⁺counterions in this example was ca. 1:1.

FIG. 19 provides a CV trace for chromium hexacyanide over a range ofoperating potentials. The data were generated using solutions of 0.05 MK₃[Cr(CN)₆] at a pH of 9, recorded at a glassy carbon electrode.

FIG. 20 provides a CV trace for manganese hexacyanide over a range ofoperating potentials. The data were generated using solutions of 0.1 MK₃[Mn(CN)₆] at a pH of 9, recorded at a glassy carbon electrode.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present disclosure may be understood more readily by reference tothe following description taken in connection with the accompanyingFigures and Examples, all of which form a part of this disclosure. It isto be understood that this disclosure is not limited to the specificproducts, methods, conditions or parameters described and/or shownherein, and that the terminology used herein is for the purpose ofdescribing particular embodiments by way of example only and is notintended to be limiting of any claimed disclosure. Similarly, unlessspecifically otherwise stated, any description as to a possiblemechanism or mode of action or reason for improvement is meant to beillustrative only, and the invention herein is not to be constrained bythe correctness or incorrectness of any such suggested mechanism or modeof action or reason for improvement. Throughout this text, it isrecognized that the descriptions refer both to methods of operating adevice and systems and to the devices and systems providing saidmethods. That is, where the disclosure describes and/or claims a methodor methods for operating a flow battery, it is appreciated that thesedescriptions and/or claims also describe and/or claim the devices,equipment, or systems for accomplishing these methods.

In the present disclosure the singular forms “a,” “an,” and “the”include the plural reference, and reference to a particular numericalvalue includes at least that particular value, unless the contextclearly indicates otherwise. Thus, for example, a reference to “amaterial” is a reference to at least one of such materials andequivalents thereof known to those skilled in the art, and so forth.

When a value is expressed as an approximation by use of the descriptor“about,” it will be understood that the particular value forms anotherembodiment. In general, use of the term “about” indicates approximationsthat can vary depending on the desired properties sought to be obtainedby the disclosed subject matter and is to be interpreted in the specificcontext in which it is used, based on its function. The person skilledin the art will be able to interpret this as a matter of routine. Insome cases, the number of significant figures used for a particularvalue may be one non-limiting method of determining the extent of theword “about.” In other cases, the gradations used in a series of valuesmay be used to determine the intended range available to the term“about” for each value. Where present, all ranges are inclusive andcombinable. That is, references to values stated in ranges include everyvalue within that range.

It is to be appreciated that certain features of the invention whichare, for clarity, described herein in the context of separateembodiments, may also be provided in combination in a single embodiment.That is, unless obviously incompatible or specifically excluded, eachindividual embodiment is deemed to be combinable with any otherembodiment(s) and such a combination is considered to be anotherembodiment. Conversely, various features of the invention that are, forbrevity, described in the context of a single embodiment, may also beprovided separately or in any sub-combination. Finally, while anembodiment may be described as part of a series of steps or part of amore general structure, each said step may also be considered anindependent embodiment in itself.

When a list is presented, unless stated otherwise, it is to beunderstood that each individual element of that list and everycombination of that list is to be interpreted as a separate embodiment.For example, a list of embodiments presented as “A, B, or C” is to beinterpreted as including the embodiments, “A,” “B,” “C,” “A or B,” “A orC,” “B or C,” or “A, B, or C.”

Electrochemical energy storage systems typically operate through theinterconversion of electrical and chemical energy. Various embodimentsof electrochemical energy storage systems include batteries, capacitors,reversible fuel cells and the like, and the present invention maycomprise any one or combination of these systems.

Unlike typical battery technologies (e.g., Li-ion, Ni-metal hydride,lead-acid, etc.), where energy storage materials and membrane/currentcollector energy conversion elements are unitized in a single assembly,flow batteries transport (e.g., via pumping) redox active energy storagematerials from storage tanks through an electrochemical stack, as inexemplary FIG. 1, which is described elsewhere herein in further detail.This design feature decouples the electrical energy storage system power(kW) from the energy storage capacity (kWh), allowing for considerabledesign flexibility and cost optimization.

In some embodiments, flow batteries according to the present disclosuremay also be described in terms of a first chamber comprising a negativeelectrode contacting a first aqueous electrolyte; a second chambercomprising a positive electrode contacting a second aqueous electrolyte,and a separator disposed between the first and second electrolytes. Theelectrolyte chambers provide separate reservoirs within the cell,through which the first and/or second electrolyte flow so as to contactthe respective electrodes and the separator. Each chamber and itsassociated electrode and electrolyte defines its correspondinghalf-cell. The separator provides several functions which include, e.g.,(1) serving as a barrier to mixing of first and second electrolytes; (2)electronically insulating to reduce or prevent short circuits betweenthe positive and negative electrodes; and (3) to provide for iontransport between the positive and negative electrolyte chambers,thereby balancing electron transport during charge and discharge cycles.The negative and positive electrodes provide a surface forelectrochemical reactions during charge and discharge. During a chargeor discharge cycle, electrolytes may be transported from separatestorage tanks through the corresponding electrolyte chambers. In acharging cycle, electrical power is applied to the system wherein theactive material contained in the second electrolyte undergoes aone-or-more electron oxidation and the active material in the firstelectrolyte undergoes a one-or-more electron reduction. Similarly, in adischarge cycle the second electrolyte is reduced and the firstelectrolyte is oxidized producing electrical power.

Various embodiments of the present invention describe flow batteries.Exemplary flow batteries suitably comprise: (a) a first aqueouselectrolyte comprising a first metal ligand coordination compound; (b) asecond aqueous electrolyte comprising a second metal ligand coordinationcompound; (c) a separator positioned between said first and secondaqueous electrolytes, the separator comprising an ionomer membrane; and(d) a mobile ion, wherein the separator has a thickness of less than 100microns and each of the first and second metal ligand coordinationcompound and the ionomer membrane have an associated net charge that isthe same sign.

In some embodiments, at least one of the first or second metal ligandcoordination compound is of the formula M(L1)_(3-x-y)(L2)_(x)(L3)_(y)^(m),

x and y are independently 0, 1, 2, or 3, such that 3-x-y is not lessthan zero;m is −5, −4, −3, −2, −1, 0, +1, +2, +3, +4, or +5; and

M is Al, Cr, Fe, or Ti; and

L1, L2, and L3 are each independently ascorbate, a catecholate, apyrogallate, lactate, gluconate, or citrate. In some cases, if x is 0, yis not 0.

Certain specific embodiments of the present invention include flowbatteries, each flow battery comprising:

a first aqueous electrolyte comprising a first ionically charged redoxactive material;a second aqueous electrolyte comprising a second ionically charged redoxactive material;a first electrode in contact with said first aqueous electrolyte;a second electrode in contact with said second aqueous electrolyte; anda separator comprising an ionomer membrane disposed between said firstand second aqueous electrolytes;wherein the sign of the net ionic charge of the first, second, or bothredox active materials matches that of the ionomer membrane.

The flow batteries may further comprise an external electrical circuitin electrical communication with the first and second electrodes, saidcircuit capable of charging or discharging the flow battery. Referenceto the sign of the net ionic charge of the first, second, or both redoxactive materials relates to the sign of the net ionic charge in bothoxidized and reduced forms of the redox active materials under theconditions of the operating flow battery. Further exemplary embodimentsprovide that (a) the first ionically charged redox active material hasan associated net positive or negative charge and is capable ofproviding an oxidized or reduced form over an electric potential in arange the negative operating potential of the system, such that theresulting oxidized or reduced form of the first redox active materialhas the same charge sign (positive or negative) as the first redoxactive material, the ionomer membrane also having a net ionic charge ofthe same sign; and (b) the second ionically charged redox activematerial has an associated net positive or negative charge and iscapable of providing an oxidized or reduced form over an electricpotential in a range of the positive operating potential of the system,such that the resulting oxidized or reduced form of the second redoxactive material has the same charge sign (positive or negative sign) asthe second redox active material, the ionomer membrane also having a netionic charge of the same sign; or both (a) and (b). These matchingcharges of the first and/or second electrolytes and the stationary phaseof the membrane, provides a selectivity such that, in individualembodiments, less than about 3%, less than about 2%, less than about 1%,less than about 0.5%, less than about 0.2%, or less than about 0.1% ofthe molar flux of ions passing through the membrane is attributable tothe first or second ionically charged redox active material. The term“molar flux of ions” refers to the amount of ions passing through theseparator membrane, balancing the charge associated with the flow ofexternal electricity/electrons. That is, the flow battery is capable ofoperating or operates with the substantial exclusion of the ionicallycharged redox active materials by the ionomer membrane.

The present disclosure includes independent embodiments of those flowbatteries wherein the sign of the net ionic charge of the first, second,or both redox active materials matches that of the ionomer membraneinclude those where one or more of the following features areindividually or collectively present:

(A) where, during the operation of the flow battery, the first or secondredox active materials comprise less than about 3% of the molar flux ofions passing through the ionomer membrane;

(B) where, the round trip current efficiency is greater than about 70%,greater than about 80%, or greater than about 90%;

(C) where the round trip current efficiency is greater than about 90%;

(D) where the sign of the net ionic charge of the first, second, or bothredox active materials is the same in both oxidized and reduced forms ofthe redox active materials and matches that of the ionomer membrane;

(E) where the ionomer membrane has a thickness of less than about 100μm, less than about 75 μm, less than about 50 μm, or less than about 250μm.

(F) where the flow battery is capable of operating at a current densityof greater than about 100 mA/cm₂ with a round trip voltage efficiency ofgreater than about 60%;

(G) where the energy density of the electrolytes is greater than about10 Wh/L, greater than about 20 Wh/L, or greater than about 30 Wh/L.

In other embodiments of these flow batteries, at least one of the firstor second redox active material or both first and second redox activematerials comprises a metal ligand coordination compound. The term“metal ligand coordination compound” is defined below. Where the firstand second redox active materials comprise first and second metal ligandcoordination compounds, respectively, the first metal ligandcoordination compound may be the same or different than the second metalligand coordination compound.

In other embodiments of these flow batteries, at least one of said firstor second redox active materials may be an organic compoundsubstantially devoid of metal.

Also as described below, one or both of the first and second redoxmaterials may exhibit substantially reversible electrochemical kinetics.Facile electrochemical kinetics, and especially reversibleelectrochemical kinetics are important for decreasing energy wastingelectrode overpotentials in both battery charge and discharge modes. Incertain embodiments, these substantially reversible electrochemicalkinetics are achievable or achieved using electrodes presenting asurface of an allotrope of carbon to the respective electrolyte. One orboth of the electrodes may present a surface of an allotrope of carbonto the respective electrolyte.

The aqueous electrolytes of these flow batteries may independently havea pH in a range of about 1 to about 13 pH units. In other independentembodiments, the pH of each of the first or second aqueous electrolytesor the pH of both the first and second aqueous electrolytes eachexhibits a pH in a range of about 7 to about 13, about 8 to about 13,about 9 to about 13, about 10 to about 13, about 10 to about 12, orabout 11. In other independent embodiments, the pH of the first aqueouselectrolyte is within about 2 pH units, about 1 pH unit, or about 0.5 pHunits of the pH of the second aqueous electrolyte. Additional embodiedranges for pH are provided below.

In specific embodiments, both the first and second ionically chargedredox active materials and their respective oxidized or reduced formsare negatively charged, and the ion selective membrane having astationary phase that also has a net negative charge, so as to beselectively permeable to cations to the substantial exclusion of thenegatively charged redox active materials. The first and second redoxactive materials and their respective oxidized or reduced forms mayindependently exhibit charges in a range of −2 to −5. The term“substantial exclusion” refers to the ability of the membrane to limitthe molar flux of ions passing through the membrane attributable to thefirst or second ionically charged redox active material to less thanabout 3% of the total ion flux during the operation of the flow battery.In related independent embodiments, the flux of ions attributable to thefirst or second ionically charged redox active material is less thanabout 5%, less than about 2%, less than about 1%, less than about 0.5%,less than about 0.2%, or less than about 0.1% of the total ion fluxduring the operation of the flow battery.

In other embodiments, both the first and second ionically charged redoxactive materials and their respective oxidized or reduced forms arepositively charged, the ion selective membrane having a stationary phasethat also has a net positive charge, so as to be selectively permeableto anions to the substantial exclusion of the positively charged redoxactive materials. The first and second redox active materials and theirrespective oxidized or reduced forms may independently exhibit chargesin a range of +2 to +5 over the respective potential ranges. The term“substantial exclusion” is as described above.

The ability to measure the molar flux of the charged redox activematerial through the membrane during the operation of the flow batterymay be conveniently measured for those systems in which each electrolytecomprises a redox active material based on a different metal such asprovided in some embodiments described here (e.g., iron in the positiveelectrolyte and titanium in the negative electrolyte). This may be doneby (a) operating such a cell at a fixed temperature (typically ambientroom, but also super-ambient, temperatures) for a prescribed period oftime (depending on the rate of flux, for example, 1 hour), (b) measuringand quantifying the amount of metal which has passed through themembrane from the source to second electrolyte (using, for example,atomic absorption spectroscopy, inductively coupled plasma, ionchromatography, or other suitable method), and (c) comparing that amountof metal ions with the amount of mobile ion which has passed through themembrane, corresponding to the total electrons which have passed overthat period of time. By measuring the flux as a function of time andtemperature, and membrane thicknesses, it is also possible to calculatethe thermodynamic parameters associated with this particular system, andpredict longevity of the system.

These flow batteries of the present invention include those capable ofor actually providing excellent round trip current efficiencies. Incertain embodiments, the flow batteries described above, when operating,exhibit a round trip current efficiency of at least 98% over astate-of-charge in a range of from about 35 to about 65%. In otherindependent embodiments, the flow batteries exhibit round trip currentefficiency of at least about 98.5, 99, 99.5, or 99.8% over astate-of-charge in a range of from about 35 to about 65%. In still otherembodiments, these efficiencies are achieved over a state-of-charge in arange of from about 40 to about 60% or about 50%.

The flow batteries of the present invention also provide superior opencircuit potentials and energy densities. In certain independentembodiments, the flow batteries of the present invention exhibit an opencircuit potential of at least about 0.8 V, at least about 1.0 V, atleast about 1.2 V, at least about 1.4 V, at least about 1.6 V, or atleast about 2 V. In other independent embodiments, the flow batteries ofthe present invention are able to provide an energy density of at least10 Wh/L, at least about 20 Wh/L, or at least about 30 Wh/L.

To this point, the various embodiments have been described mainly interms of individual flow batteries. It should be appreciated that, wherepossible, the descriptions should be read as including flow batteriesthat are operating or capable of operating with the specifiedcharacteristics. Similarly, the descriptions should be read as includingsystems of flow batteries, wherein the system comprises at least two ofthe flow batteries described herein.

An exemplary flow battery is shown in FIG. 1. As shown in that figure, aflow battery system may include an electrochemical cell that features a(e.g., a membrane) that separates the two electrodes of theelectrochemical cell. Electrode 10 is suitably a conductive material,such as a metal, carbon, graphite, and the like. Tank 50 may containfirst redox material 30, which material is capable of being cycledbetween an oxidized and reduced state.

A pump 60 may effect transport of the first active material 30 from thetank 50 to the electrochemical cell. The flow battery also suitablyincludes a second tank (not labeled) that contains the second activematerial 40. The second active material 40 may or may not be the same asactive material 30. A second pump (not labeled) may effect transport ofsecond redox material 40 to the electrochemical cell. Pumps may also beused to effect transport of the active materials from theelectrochemical cell to the tanks of the system. Other methods ofeffecting fluid transport—e.g., siphons—may be used to transport redoxmaterial into and out of the electrochemical cell. Also shown is a powersource or load 70, which completes the circuit of the electrochemicalcell and allows the user to collect or store electricity duringoperation of the cell.

It should be understood that FIG. 1 depicts a specific, non-limitingembodiment of a flow battery. Accordingly, devices according to thepresent disclosure may or may not include all of the aspects of thesystem depicted in FIG. 1. As one example, a system according to thepresent disclosure may include active materials that are solid, liquid,or gas and/or solids, liquids, or gases dissolved in solution, orslurries. Active materials may be stored in a tank, in a vessel open tothe atmosphere, or simply vented to the atmosphere.

In some cases, a user may desire to provide higher charge or dischargevoltages than available from a single battery. In such cases, and incertain embodiments, then, several batteries are connected in seriessuch that the voltage of each cell is additive. An electricallyconductive, but non-porous material (e.g., a bipolar plate) may beemployed to connect adjacent battery cells in a bipolar stack, whichallows for electron transport but prevents fluid or gas transportbetween adjacent cells. The positive electrode compartments and negativeelectrode compartments of individual cells are suitably fluidicallyconnected via common positive and negative fluid manifolds in the stack.In this way, individual electrochemical cells can be stacked in seriesto yield a voltage appropriate for DC applications or conversion to ACapplications.

A region of a cell in a stack will represent a differential element (forexample 2-60 cm²) of a larger cell, which has practical areas ofapproximately 200 to 6000 cm² for useful devices. This differentialelement will be characterized by uniform conditions across that area,which includes positive and negative active material and electrolyteconcentrations, voltage, and current density. A cell is represented bythe entire active area range given above, where non-uniformities mayexist in the active material and electrolyte concentrations, voltages,and current density.

In additional embodiments, the cells, cell stacks, or batteries areincorporated into larger energy storage systems, suitably includingpiping and controls useful for operation of these large units. Piping,control, and other equipment suitable for such systems are known in theart, and include, for example, piping and pumps in fluid communicationwith the respective electrochemical reaction chambers for movingelectrolytes into and out of the respective chambers and storage tanksfor holding charged and discharged electrolytes. The energy storage andgeneration systems described by the present disclosure may also includeelectrolyte circulation loops, which loops may comprise one or morevalves, one or more pumps, and optionally a pressure equalizing line.The energy storage and generation systems of this disclosure can alsoinclude an operation management system. The operation management systemmay be any suitable controller device, such as a computer ormicroprocessor, and may contain logic circuitry that sets operation ofany of the various valves, pumps, circulation loops, and the like.

A suitable flow battery system may comprise a flow battery (including acell or cell stack); storage tanks and piping for containing andtransporting the electrolytes; control hardware and software (which mayinclude safety systems); and a power conditioning unit. The flow batterycell stack accomplishes the conversion of charging and dischargingcycles and determines the peak power of energy storage system, whichpower may in some embodiments be in the kW range. The storage tankscontain the positive and negative active materials; the tank volumedetermines the quantity of energy stored in the system, which may bemeasured in kWh. The control software, hardware, and optional safetysystems suitably include sensors, mitigation equipment and otherelectronic/hardware controls and safeguards to ensure safe, autonomous,and efficient operation of the flow battery energy storage system. Suchsystems are known to those of ordinary skill in the art. A powerconditioning unit may be used at the front end of the energy storagesystem to convert incoming and outgoing power to a voltage and currentthat is optimal for the energy storage system or the application. Forthe example of an energy storage system connected to an electrical grid,in a charging cycle the power conditioning unit would convert incomingAC electricity into DC electricity at an appropriate voltage and currentfor the electrochemical stack. In a discharging cycle, the stackproduces DC electrical power and the power conditioning unit converts toAC electrical power at the appropriate voltage and frequency for gridapplications.

The energy storage systems of the present disclosure are, in someembodiments, suited to sustained charge or discharge cycles of severalhour durations. As such, the systems of the present disclosure may beused to smooth energy supply/demand profiles and provide a mechanism forstabilizing intermittent power generation assets (e.g., from renewableenergy sources). It should be appreciated, then, that variousembodiments of the present disclosure include those electrical energystorage applications where such long charge or discharge durations arevaluable. For example, non-limiting examples of such applicationsinclude those where systems of the present disclosure are connected toan electrical grid include, so as to allow renewables integration, peakload shifting, grid firming, baseload power generation consumption,energy arbitrage, transmission and distribution asset deferral, weakgrid support, frequency regulation, or any combination thereof. Cells,stacks, or systems according to the present disclosure may be used toprovide stable power for applications that are not connected to a grid,or a micro-grid, for example as power sources for remote camps, forwardoperating bases, off-grid telecommunications, remote sensors, or anycombination thereof.

Flow battery energy storage efficacy is determined by both the roundtrip DC-DC energy efficiency (RT_(EFF)) and the energy density of theactive materials (measured in Wh/L). The RT_(EFF) is a composite ofvoltage and current efficiencies for both the battery charge anddischarge cycles. In electrochemical devices, voltage and currentefficiencies are functions of the current density, and while voltage andcurrent efficiency typically decrease as current density (mA/cm²)increases, high current densities are often desirable to reduceelectrochemical stack size/cost used to achieve a given power rating.Active material energy density is directly proportional to the cell OCV(OCV=open circuit voltage), the concentration of active species, and thenumber of electrons transferred per mole of active species. High energydensities are desirable to reduce the volume of active materialsrequired for a given quantity of stored energy.

It should be appreciated that, while the various embodiments describedherein are described in terms of flow battery systems, the samestrategies, designs, chemical embodiments, and combinations thereof, mayalso be employed with stationary (non-flow) electrochemical cells,batteries, or systems, including those where one or both half cellsemploy stationary electrolytes. Each of these embodiments is consideredwithin the scope of the present invention.

TERMS

Throughout this specification, words are to be afforded their normalmeaning, as would be understood by those skilled in the relevant art.However, so as to avoid misunderstanding, the meanings of certain termswill be specifically defined or clarified.

The term “active material” is well known to those skilled in the art ofelectrochemistry and electrochemical energy storage and is meant torefer to materials which undergo a change in oxidation state duringoperation of the system. Active materials may comprise a solid, liquid,or gas and/or solids, liquids, or gasses dissolved in solution. In someembodiments, active materials comprise molecules, supramolecules, or anycombination thereof, dissolved in solution. The concentration of theseactive materials may be greater than 2 M, between 1 and 2 M, about 1.5M, between 0.5 M and 1 M, or even less than 0.5 M.

Suitable active material may comprise a “metal ligand coordinationcompound.” Suitable metal ligand coordination compounds are known tothose skilled in the art of electrochemistry and inorganic chemistry. Ametal ligand coordination compound may comprise a metal ion bonded to anatom or molecule. The bonded atom or molecule is referred to as a“ligand”. In certain non-limiting embodiments, the ligand may comprise amolecule comprising C, H, N, and/or O atoms. In other words, the ligandmay comprise an organic molecule. The metal ligand coordinationcompounds of the present disclosure are understood to comprise at leastone ligand that is not water, hydroxide, or a halide (F⁻, Cl⁻, Br⁻, I⁻).

Metal ligand coordination compounds may comprise a “redox active metalion” and/or a “redox inert metal ion”. The term “redox active metal ion”is intended to connote that the metal undergoes a change in oxidationstate under the conditions of use. As used herein, the term “redoxinert” metal ion is intended to connote that the metal does not undergoa change in oxidation state under the conditions of use. Metal ions maycomprise non-zero valence salts of, e.g., Al, Ca, Co, Cr, Sr, Cu, Fe,Mg, Mn, Mo, Ni, Pd, Pt, Ru, Sn, Ti, Zn, Zr, V, U or a combinationthereof. The skilled artisan would be able to recognize thecircumstances where a given non-zero valence metal would be redox activeor inactive under the prescribed electrolyte environments.

Suitable active materials may comprise an “organic active material”. Anorganic active material may comprise a molecule or supramolecule thatdoes not contain a transition metal ion. It is further understood thatorganic active materials are meant to comprise molecules orsupramolecules that are dissolved in aqueous solution. Suitable organicactive materials are capable of undergoing a change in oxidation stateduring operation of the electrochemical energy storage system.Accordingly, the molecule or supramolecule may accept or donate anelectron during operation of the system.

The terms “a catecholate,” “a glycolate,” “a polyol”, “ahydroxyalkanoate”, “a benzoate”, “a phthalate”, “a urea” and “apolyamine” reflect the fact that these ligands may be optionallysubstituted with at least one group independently selected from H, C₁₋₆alkoxy, C₁₋₆ alkyl, C₁₋₆ alkenyl, C₁₋₆ alkynyl, 5-6 membered aryl orheteroaryl, a boric acid or a salt thereof, carboxy acid or a saltthereof, cyano, halo, hydroxyl, nitro, sulfonate, sulfonic acid or asalt thereof, phosphonate, phosphonic acid or a salt thereof, or apolyglycol (preferably polyethylene glycol).

Alkaneoate includes alpha, beta, and gamma forms. Polyamine includes,but is not limited to, ethylene diamine, ethylene diamine tetraaceticacid (EDTA), and diethylene triamine pentaacetic acid (DTPA).Catecholeate includes all compositions comprising a 1,2-dihydroxybenzenemoiety. Such moieties include hydroxycatecholates (includingpyrogallate), as well as substitutents listed herein. Substituentsinclude, but are not limited to, alkyl, alkenyl, and alkynyl (each referto branched or linear structures and structures optionally substitutedwith one or more carboxyl, halo, hydroxyl or other electron withdrawingor electron donating groups. Substituents also include 5-6 membered arylor heteroaryls include phenyl, pyridinyl, furyl, pyrrolyl, imidazolyl,triazole, or thiophenyl. Electron withdrawing or donating substituentscan be added to the periphery of the aromatic rings to modulate theredox potential of the redox active ligands.

Unless otherwise specified, the term “aqueous” refers to a solventsystem comprising at least about 98% by weight of water, relative tototal weight of the solvent. In some applications, soluble, miscible, orpartially miscible (emulsified with surfactants or otherwise)co-solvents may also be usefully present which, for example, extend therange of water's liquidity (e.g., alcohols/glycols). When specified,additional independent embodiments include those where the “aqueous”solvent system comprises at least about 55%, at least about 60 wt %, atleast about 70 wt %, at least about 75 wt %, at least about 80%, atleast about 85 wt %, at least about 90 wt %, at least about 95 wt %, orat least about 98 wt % water, relative to the total solvent. It somesituations, the aqueous solvent may consist essentially of water, and besubstantially free or entirely free of co-solvents or other species. Thesolvent system may be at least about 90 wt %, at least about 95 wt %, orat least about 98 wt % water, and, in some embodiments, be free ofco-solvents or other species.

In addition to the redox active materials described below, the aqueouselectrolytes may contain additional buffering agents, supportingelectrolytes, viscosity modifiers, wetting agents, and the like.

The term “bipolar plate” refers to an electrically conductive,substantially nonporous material that may serve to separateelectrochemical cells in a cell stack such that the cells are connectedin series and the cell voltage is additive across the cell stack. Thebipolar plate has two surfaces such that one surface of the bipolarplate serves as a substrate for the positive electrode in one cell andthe negative electrode in an adjacent cell. The bipolar plate typicallycomprises carbon and carbon containing composite materials.

The term “cell geometry” is well known to those of ordinary skill in theart of electrochemistry and refers to the over physical construction ofthe flow battery.

The term “cell mechanical loading” is well known to those of ordinaryskill in the art of electrochemistry and refers to the degree ofmechanical compression that is experienced in an individual flow batterycell or, on an average basis by an individual cell in a stack of cells.The degree of mechanical compression is normally measured in psi.

The term “cell potential” is readily understood by those skilled in theart of electrochemistry and is defined to be the voltage of theelectrochemical cell during operation. The cell potential may be furtherdefined by Equation 1:

Cell Potential=OCV−η _(pos)−η_(neg) −iR  (1)

where OCV is the “open circuit potential”, η_(pos) and η_(neg) are theoverpotentials for the positive and negative electrodes at a givencurrent density, respectively, and iR is the voltage loss associatedwith all cell resistances combined. The “open circuit potential” or OCVmay be readily understood according to Equation 2:

OCV=E ⁺ −E ⁻  (2)

where E⁺ and E⁻ are the “half-cell potentials” for the redox reactionstaking place at the positive and negative electrodes, respectively. Thehalf-cell potentials may be further described by the well-known NernstEquation 3:

E=E ⁰−RT/nF ln(X _(red) /X _(ox))  (3)

wherein E⁰ is the standard reduction potential for redox couple ofinterest (e.g. either the positive or negative electrode), the R is theuniversal gas constant, T is temperature, n is the number of electronstransferred in the redox couple of interest, F is Faraday's constant,and X_(red)/X_(ox) is the ratio of reduced to oxidized species at theelectrode.

The term “current density” refers to the total current passed in anelectrochemical cell divided by the geometric area of the electrodes ofthe cell and is commonly reported in units of mA/cm².

The term “current efficiency” (I_(EFF)) may be described as the ratio ofthe total charge produced upon discharge of the system to the totalcharge passed upon charge. In some embodiments, the charge produced ondischarge or passed on charge can be measured using standardelectrochemical coulomb counting techniques well known to those ofordinary skill in the art. Without being bound by the limits of anytheory, the current efficiency may be a function of the state of chargeof the flow battery. In some non-limiting embodiments the currentefficiency can be evaluated over an SOC range of about 35% to about 60%.

The term “diffusion media properties” is well known to those of ordinaryskill in the art of electrochemistry and refers to the properties of amaterial that allow ions or molecules to diffuse across that material.

The term “energy density” refers to the amount of energy that may bestored, per unit volume, in the active materials. Energy density, asused herein, refers to the theoretical energy density of energy storageand may be calculated by Equation 4:

Energy density=(26.8 A-h/mol)×OCV×[e ⁻]  (4)

where OCV is the open circuit potential at 50% state of charge, asdefined above, (26.8 A-h/mol) is Faraday's constant, and [e⁻] is theconcentration of electrons stored in the active material at 99% state ofcharge. In the case that the active materials largely comprise an atomicor molecular species for both the positive and negative electrolyte,[e⁻] may be calculated as:

[e ⁻]=[active materials]×n/2  (5)

where [active materials] is the concentration (mol/L or M) of the activematerial in either the negative or positive electrolyte, whichever islower, and n is the number of electrons transferred per molecule ofactive material. The related term “charge density” refers to the totalamount of charge that each electrolyte may contain. For a givenelectrolyte:

Charge density=(26.8 A-h/mol)×[active material]×n  (6)

where [active material] and n are as defined above.

The term “energy efficiency” may be described as the ratio of the totalenergy produced upon discharge of the system to the total energyconsumed upon charge. The energy efficiency (RT_(EFF)) may be computedby Equation 7:

RT_(EFF) =V _(EFF,RT) ×I _(EFF)  (7)

As used herein, the term “evolution current” describes the portion ofthe electrical current applied in an energized flow batteryconfiguration which is associated with the evolution (generation) of aparticular chemical species. In the current context, then, when asufficient overpotential vide infra) is applied in a flow battery suchthat either or both oxygen evolves at the positive electrode or hydrogenevolves at the negative electrode, that portion of the currentassociated with the evolution of oxygen or hydrogen is the oxygenevolution current or hydrogen evolution current, respectively.

In certain preferred embodiments, there is no current associated withhydrogen evolution, oxygen evolution, or both hydrogen and oxygenevolution. This may occur when the positive half-cell is operating at apotential less than the thermodynamic threshold potential or thethreshold overpotential of the positive electrode (i.e., no oxygenproduced; see explanation of terms below) or the negative half-cell cellis operating at a potential more positive than the thermodynamicthreshold potential or the threshold overpotential of the negativeelectrode (i.e., no hydrogen produced), or both. In separateembodiments, the batteries operate within 0.3 V, within 0.25 V, within0.2 V, within 0.15 V, or within 0.1 V of either the thermodynamicthreshold potential or the threshold overpotential of the respectivepositive or negative electrodes.

In embodiments wherein gas is evolved, the portion of current associatedwith gas evolution (either hydrogen or oxygen or both) is suitably lessthan about 20%, less than about 15%, less than about 10%, less thanabout 5%, less than about 2%, or less than about 1% of the total appliedcurrent. Lower gas evolution currents are considered particularlysuitable for battery (cell or cell stack) efficiencies.

The term “excluding” refers to the ability of a separator to not allowcertain ions or molecules to flow through the separator and typically ismeasured as a percent.

The term “mobile ion” is understood by those skilled in the art ofelectrochemistry and is meant to comprise the ion which is transferredbetween the negative and positive electrode during operation of theelectrochemical energy storage system. The term “mobile ion” may alsorefer to as an ion that carries greater than at least 80% of the ioniccurrent during charger/discharge.

As used herein, the terms “negative electrode” and “positive electrode”are electrodes defined with respect to one another, such that thenegative electrode operates or is designed or intended to operate at apotential more negative than the positive electrode (and vice versa),independent of the actual potentials at which they operate, in bothcharging and discharging cycles. The negative electrode may or may notactually operate or be designed or intended to operate at a negativepotential relative to the reversible hydrogen electrode. The negativeelectrode is associated with the first aqueous electrolyte and thepositive electrode is associated with the second electrolyte, asdescribed herein.

The term “overpotential” is well understood by those skilled in the artof electrochemistry and is defined by the difference in voltage betweenan electrode during operation of an electrochemical cell and the normalhalf-cell potential of that electrode, as defined by the Nernstequation. Without being bound by theory, the term overpotential is meantto describe the energy, in excess of that required by thermodynamics, tocarry out a reaction at a given rate or current density. The term“overpotential” also describes a potential more positive than thethermodynamic onset voltage for oxygen evolution from water at thepositive electrode and more negative than the thermodynamic onsetvoltage for hydrogen evolution from water at the negative electrode.

Similarly, as used herein, the term “threshold overpotential” refers tothe overpotential at which either hydrogen or oxygen gas begins toevolve at the respective electrode. Note that an electrochemical systemcomprising “imperfect” (i.e., less than ideal catalytically) electrodescan be operated in three regions: (a) at a potential “below” thethermodynamic onset potential (i.e., more positive than thethermodynamic onset potential of the negative electrode and morenegative than the thermodynamic onset potential of the positiveelectrode; no gas evolving so no gas evolution current); (b) at apotential between the thermodynamic threshold potential and thresholdoverpotential (no gas evolving and still no evolution current); and (c)beyond the threshold overpotential (gas evolving and exhibiting a gasevolution current). Such threshold overpotentials can be identified bythose skilled in the art for a given system, for example, by measuringgas evolution as a function of applied half-cell potential (using e.g.,a mass spectrometer), in the presence or absence of an electroactivematerial. See also below.

The gas evolution threshold potentials are also affected by the natureof the electrolytes. Certain chemicals are known to inhibit theevolution of hydrogen and oxygen in electrolytic cells, either becauseof some activity in the bulk electrolyte or because of their ability tocoat or otherwise deactivate their respective electrodes; for example,macromolecules or oligomers or salts, such as chloride or phosphate, onPt surfaces. Accordingly, in certain embodiments, then, either the firstor second or both first and second electrolytes comprise at least onecompound increases the hydrogen or oxygen threshold overpotential of thesystem, respectively.

As used herein, the terms “regenerative fuel cell” or “reversible fuelcell” or “flow battery” or “flow energy device” connote the same orsimilar type of device, which utilizes the same battery configuration(including cell or cell stack) for both energy storage and energygeneration.

The term “reversible hydrogen electrode,” or RHE, is used in itsconventional meaning. That is, a reversible hydrogen electrode (RHE) isa reference electrode. The potential of the RHE, E(RHE) corresponds tothe potential for the reaction of Equation 8:

2H⁺⇄H₂  (8)

When the reaction of Equation 8 is carried out at equilibrium at a givenpH and 1 atm H₂. This potential can be reference to a normal hydrogenelectrode, E(NHE), by the following relation:

E(RHE)=E(NHE)−0.059×pH=0.0 V−0.059×pH  (9)

where E(NHE) is the potential for the normal hydrogen electrode(NHE=0.0V), defined as the potential for the reaction of Equation 8 atstandard state (1M H⁺, 1 atm H₂). Thus a potential of 0 V vs. RHEcorresponds to a voltage of 0 V vs. NHE at pH 0 and −0.413 V vs. NHE atpH 7.

The term “selectivity” is well known to those of ordinary skill in theart of electrochemistry and refers to the ability of a membrane to allowa ratio of the movement of mobile ions to active materials through amembrane. For example, a membrane that allows a 50:1 ratio of mobileions to active materials to pass through would have a selectivity of 50.

The terms “separator” and “membrane” refer to an ionically conductive,electrically insulating material disposed between the positive andnegative electrode of an electrochemical cell.

The polymer electrolytes useful in the present disclosure may be anionor cation conducting electrolytes. Where described as an “ionomer,” theterm refers to a polymer comprising both electrically neutral and afraction of ionized repeating units, wherein the ionized units arependant and covalently bonded to the polymer backbone. The fraction ofionized units may range from about 1 mole percent to about 90 molepercent, but may be further categorized according to their ionized unitcontent. For example, in certain cases, the content of ionized units areless than about 15 mole percent; in other cases, the ionic content ishigher, typically greater than about 80 mole percent. In still othercases, the ionic content is defined by an intermediate range, forexample in a range of about 15 to about 80 mole percent. Ionized ionomerunits may comprise anionic functional groups comprising sulfonate,carboxylate, and the like. These functional groups can be chargebalanced by, mono-, di-, or higher-valent cations, such as alkali oralkaline earth metals. Ionomers may also include polymer compositionscontaining attached or embedded quaternary ammonium, sulfonium,phosphazenium, and guanidinium residues or salts. The polymers useful inthe present disclosure may comprise highly fluorinated or perfluorinatedpolymer backbones. Certain polymer electrolytes useful in the presentdisclosure include copolymers of tetrafluoroethylene and one or morefluorinated, acid-functional co-monomers, which are commerciallyavailable as NAFION™ perfluorinated polymer electrolytes from DuPontChemicals, Wilmington Del. Other useful perfluorinated electrolytescomprise copolymers of tetrafluoroethylene (TFE) andFSO₂—CF₂CF₂CF₂CF₂—O—CF═CF₂.

The term “stack” or “cell stack” or “electrochemical cell stack” refersto a collection of individual electrochemical cells that are inelectrically connected. The cells may be electrically connected inseries or in parallel. The cells may or may not be fluidly connected.

The term “state of charge” (SOC) is determined from the concentrationratio of reduced to oxidized species at an electrode (X_(red)/X_(ox)).For example, in the case of an individual half-cell, when X_(red)=X_(ox)such that X_(red)X_(ox)=1, the half-cell is at 50% SOC, and thehalf-cell potential equals the standard Nerstian value, E°. When theconcentration ratio at the electrode surface corresponds toX_(red)/X_(ox)=0.25 or X_(red)/X_(ox)=0.75, the half-cell is at 25% and75% SOC respectively. The SOC for a full cell depends on the SOCs of theindividual half-cells and in certain embodiments the SOC is the same forboth positive and negative electrodes. Measurement of the cell potentialfor a battery at OCV, and using Equations 2 and 3 the ratio ofX_(red)/X_(ox) at each electrode can be determined, and therefore theSOC for the battery system.

The term “supporting electrolyte” is well-known in the arts ofelectrochemistry and energy storage, and is intended to refer to anyspecies which is redox inactive in the window of electric potential ofinterest and aids in supporting charge and ionic conductivity. In thepresent case, a supporting electrolyte does not substantially compromisethe solubility of the coordination complex. Non-limiting examplesinclude salts comprising an alkali metal, ammonium ion including anammonium ion partially or wholly substituted by alkyl or aryl groups,halide (e.g., Cr⁻, Br⁻, I⁻), chalcogenide, phosphate, hydrogenphosphate, phosphonate, nitrate, sulfate, nitrite, sulfite, perchlorate,tetrafluoroborate, hexafluorophosphate, or a mixture thereof, and othersknown in the art.

The term “voltage efficiency” may be described as the ratio of theobserved electrode potential, at a given current density, to thehalf-cell potential for that electrode (×100%), wherein the half-cellpotential is calculated as described above. Voltage efficiencies can bedescribed for a battery charging step, a discharging step, or a “roundtrip voltage efficiency”. The round trip voltage efficiency (V_(EFF,RT))at a given current density can be calculated from the cell voltage atdischarge (V_(Discharge)) and the voltage at charge (V_(charge)) usingEquation 10:

V _(EFF,RT) =V _(Discharge) /V _(Charge)×100%  (10)

Exemplary Operating Characteristics

The present disclosure provides a variety of technical features of thedisclosed systems and methods. It should be understood that any one ofthese features may be combined with any one or more other features. Forexample, a user might operate a system featuring an electrolyte thatincludes an organic active material (e.g., a quinone), wherein thatelectrode has a pH of about 3. Such a system might also feature amembrane separator having a thickness of about 35 micrometers. It shouldbe further understood that the present disclosure is not limited to anyparticular combination or combinations of the following features.

Certain embodiments of the present invention provides method ofoperating a flow battery, each method comprising charging said batteryby the input of electrical energy or discharging said battery by theremoval of electrical energy. Further embodiments provide applying apotential difference across the first and second electrode, with anassociated flow of electrons, so as to: (a) reduce the first redoxactive material while oxidizing the second redox active material; or (b)oxidize the first redox active material while reducing the second redoxactive material. Complementary methods provide those where each methodcomprises applying a potential difference across the first and secondelectrode, with an associated flow of electrons, so as to: (a) oxidizethe first redox active metal-ligand coordination compound; or (b) reducethe second redox active metal-ligand coordination compound; or (c) both(a) and (b).

Mobile ions typically include proton, hydronium, or hydroxide. Invarious embodiments of the present disclosure, one may additionallytransport ions other than proton, hydronium, or hydroxide (e.g., whenthese ions are present in comparatively low concentration, such as below1 M). Separate embodiments of these methods of operating a flow batteryinclude those wherein the mobile ion does not consist essentially ofprotons, hydronium, or hydroxide. In this embodiment, less than 50% ofthe mobile ions comprise protons, hydronium, or hydroxide. In otherembodiments, less than about 40%, less than about 30%, less than about20%, less than about 10%, less than about 5%, or less than about 2% ofthe mobile ions comprise protons, hydronium, or hydroxide. Exemplarymobile ions in these embodiments include alkali metal or alkaline earthmetal cations (especially Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺), halides(e.g., F⁻, Cl⁻, or Br⁻), or OH⁻.

In some embodiments of the present disclosure, it is advantageous tooperate between pH 1 and 13 (e.g. to enable active material solubilityand/or low system cost). Accordingly, one or both electrolytes can becharacterized as having a pH in the range of from about 1 to about 13,or between about 2 and about 12, or between about 4 and about 10, oreven between about 6 and about 8. In other embodiments, at least one ofthe electrolytes has a pH in a range of from about 9 to about 13, fromabout 8 to about 12, from about 10 to about 12, or from 10.5 to about11.5. For the most part, the compounds described herein comprisingcatecholate or pyrogallate are stable and operable at pH's within eachof the ranges described herein. Generally, the compounds describedherein are stable and operable at pH's within each of these ranges. Insome embodiments, the pH of the electrolyte may be maintained by abuffer. Typical buffers include salts of phosphate, borate, carbonate,silicate, tris(hydroxymethyl)aminomethane (Tris),4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES),piperazine-N,N′-bis(ethanesulfonic acid) (PIPES), and combinationsthereof. A user may add an acid (e.g., HCl, HNO₃, H₂SO₄ and the like), abase (NaOH, KOH, and the like), or both to adjust the pH of a givenelectrolyte as desired.

The pH of the first and second electrolytes may suitably be equal orsubstantially similar; in other embodiments, the pH of the twoelectrolytes differ by a value in the range of about 0.1 to about 2 pHunits, about 1 to about 10 pH units, about 5 to about 12 pH units, about1 to about 5 pH units, about 0.1 to about 1.5 pH units, about 0.1 toabout 1 pH units, or about 0.1 to about 0.5 pH units. In this context,the term “substantially similar,” without further qualification, isintended to connote that the difference in pH between the twoelectrolytes is less than about 1 pH unit. Additional optionalembodiments provide that the pH difference is less than about 0.4, lessthan about 0.3, less than about 0.2, or less than about 0.1 pH units.

The disclosed systems and methods may also comprise active materials andmembrane ionomers which are charged. The term “charge” in refers to the“net charge” or total charge associated with an active material orionomer moiety. The charged species may be anionic or cationic. Incertain desired embodiments of the present disclosure it is advantageousfor the active materials and membrane ionomers to comprise charges ofthe same sign (e.g. to prevent transfer of the active material acrossthe membrane).

Systems and methods according to the present disclosure also featureactive materials comprising metal-ligand coordination compounds.Metal-ligand coordination compounds may be present at, e.g., aconcentration of at least about 0.25 M, at least about 0.35 M, at leastabout 0.5 M, at least about 0.75 M, at least about 1 M, at least about1.25 M, at least about 1.5 M, at least about 2 M, or greater than 2 M.

The metal-ligand coordination compound may be further characterized withrespect to the nature of the oxidizable or reducible species. Forexample, in some cases, the redox potential of the metal-ligandcoordination compound may be defined by transitions entirely within themetal center—i.e., the redox potential is defined by the accessibilityof and energies associated with transitions between various valencestates within the metal. In other cases, the oxidation/reduction may belocalized within the ligand system. In still other cases, theoxidation/reduction may be distributed throughout the entire redoxactive complex, such that both the metal and the ligand system sharingin the distribution of charge. Preferably, the redox potential shoulddiffer by at least 0.5 volt. More preferably, the redox potential shoulddiffer by at least 1.0 volt. It is suitable for each electrolyte tocontain the same metal center, so long as the first metal center andsecond metal center have different oxidation states.

In particular embodiments of the present disclosure, the metal-ligandcoordination compound may comprise ligands which are mono-, bi-, tri-,or multidentate. Monodentate ligands bind to metals through one atom,whereas bi-, tri-, or multidentate ligands bind to metals through 2, 3,or more atoms, respectively. Examples of monodentate ligands includehalogens (F⁻, Cr⁻, Br⁻, I⁻), cyanide (CN), carbonyl or carbon monoxide(CO), nitride (N³⁻), oxo (O²⁻), hydroxo (OH), water (H₂O), sulfide(S²⁻), pyridine, pyrazine, and the like. Other types of ligand bondingmoieties include amino groups (NR₃), amido groups (N(R)₂), imido groups(NR), alkoxy groups (R—CO⁻), siloxy (R—SiO⁻), thiolate (R—S⁻), and thelike, which may comprise mono-, bi-, tri-, or multidentate ligands.Examples of bidentate ligands include catechol, bipyridine, bipyrazine,ethylenediamine, diols (including ethylene glycol), and the like.Examples of tridentate ligands include terpyridine, diethylenetriamine,triazacyclononane, trisaminomethane, and the like. Other acceptableligands include quinones, hydroquinones, viologens, pyridinium,acridinium, polycyclic aromatic hydrocarbons and combinations thereof.

In other embodiments, the first or second redox active material, or boththe first and second redox active materials comprise a metal ligandcoordination compound of the formula M(L1)_(3-x-y)(L2)_(x)(L3)_(y) ^(m),where M is independently a non-zero valent metal or metalloid of Groups2-16, including lanthanides and actinides,

where x and y are independently 0, 1, 2, or 3, such that 3-x-y is notless than zero;m is independently −5, −4, −3, −2, −1, 0, 1, 2, 3, 4, or 5; andL1, L2, and L3 are each independently ascorbate, citrate, gluconate,lactate, or a compound having structure according to Formula I, or anoxidized or reduced form thereof:

whereinAr is a 5-20 membered aromatic moiety, optionally comprising one of morering O, N, or S heteroatoms;X₁ and X₂ are independently —OH, —NHR₂, —SH, or an anion thereof, X₁ andX₂ being positioned ortho to one another;R₁ is independently at each occurrence H, C₁₋₆ alkoxy, C₁₋₆ alkyl, C₁₋₆alkenyl, C₁₋₆ alkynyl, 5-6 membered aryl or heteroaryl, a boric acid ora salt thereof, carboxy acid or a salt thereof, carboxylate, cyano,halo, hydroxyl, nitro, sulfonate, sulfonic acid or a salt thereof,phosphonate, phosphonic acid or a salt thereof, or a polyglycol(preferably polyethylene glycol);R₂ is independently H or C₁₋₃ alkyl; andn is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.

In other embodiments, the first redox active material comprises a metalligand coordination complex of the formula M(L1)_(3-x-y)(L2)_(x)(L3)_(y)^(m) and x and y are independently 0, 1, 2, or 3, such that 3-x-y is notless than zero;

m is −5, −4, −3, −2, −1, 0, 1, 2, 3, 4, or 5; and

M is Al, Cr, Fe, or Ti; and

L1, L2, and L3 are each independently ascorbate, a catecholate, apyrogallolate, lactate, gluconate, or citrate. The terms “a catecholate”and “a pyrogallolate” reflect the fact that these ligands may beoptionally substituted with at least one R₁ group, as definedabove—i.e., in independent embodiments, the catecholate or pyrogallateare substituted and unsubstituted.

Some embodiments provide certain separator characteristics, both inabsolute compositional and parametric terms and in relation to the metalligand coordination compounds. Other embodiments describe specificfunctional characteristics which derive from the inventive systems.

In still other embodiments, the second redox active material comprises ametal ligand coordination complex of the formulaM(L1)_(3-x-y)(L2)_(x)(L3)_(y) ^(m),

M comprises Al, Ca, Ce, Co, Cr, Fe, Mg, Mo, S, Sn, Ti, U, W, Zn, or Zr;L1, L2, and L3 are each independently ascorbate, a catecholate, apyrogallate, lactate, gluconate, or citrate;x and y are independently 0, 1, 2, or 3, such that 3-x-y is not lessthan 0;and m is −2, −3, −4, or −5. Related embodiments provide that if x is 0,y is not 0.

Either or both of the electrodes that carry out the electrochemicalreactions may comprise carbon and either or both of the first and secondmetal ligand coordination compound independently exhibits substantiallyreversible electrochemical kinetics. Similarly, in either case, separateindependent embodiments provide that if x is 0, y is not 0.

In those embodiments where the first and second aqueous electrolyteseach comprises a first and second metal ligand coordination compound,respectively, the first and second metal ligand coordination compoundsmay be the same or different, though preferably they are different.

The invention also provides those embodiments were either the first orthe second or both the first and second metal ligand coordinationcompound comprises at least one ligand having a structure according toFormula I. Similarly, either or both of the metal ligand coordinationcompounds may comprise at least one ligand having a structure accordingto Formula IA, IB, or IC:

whereinX₁ and X₂ are independently —OH, —NHR₂, —SH, or an anion thereof;R₁ is independently H, C₁₋₃ alkoxy, C₁₋₃ alkyl, a boric acid or a saltthereof, a boric acid or a salt thereof, carboxy acid or a salt thereof,carboxylate, cyano, halo, hydroxyl, nitro, sulfonate, sulfonic acid or asalt thereof, phosphonate, phosphonic acid or a salt thereof, or apolyglycol (preferably polyethylene glycol)R₂ is independently H or C₁₋₃ alkyl; andn is 0, 1, 2, 3, or 4.

Additional embodiments provide either or both of the metal ligandcoordination compounds comprises at least one ligand having a structureaccording to Formula IA, IB, or IC, but where:

X₁ and X₂ are both OH or an anion thereof;R₁ is independently H, C₁₋₃ alkoxy, a boric acid or a salt thereof, aboric acid or a salt thereof, carboxy acid or a salt thereof,carboxylate, cyano, halo, hydroxyl, nitro, sulfonate, sulfonic acid or asalt thereof, phosphonate, phosphonic acid or a salt thereof, or apolyglycol (preferably polyethylene glycol); andn is 1.

In various embodiments, either each or both of the first or second metalligand coordination compound may also comprise at least one ascorbate, acatecholate, citrate, gluconate, lactate, or a pyrogallate ligand.

The disclosed systems and methods may feature electrochemical cellseparators and/or membranes that have certain characteristics. In thisdisclosure, the terms membrane and separator are used interchangeably.The membranes of the present disclosure may, in some embodiments,feature a membrane separator having a thickness of less than about 500micrometers, less than about 300 micrometers, less than about 250micrometers, less than about 200 micrometers, less than about 100micrometers, less than about 75 micrometers, less than about 50micrometers, less than about 30 micrometers, less than about 25micrometers, less than about 20 micrometers, less than about 15micrometers, or less than about 10 micrometers. Suitable separatorsinclude those separators in which the flow battery is capable ofoperating with a current efficiency of greater than about 85% with acurrent density of 100 mA/cm² when the separator has a thickness of 100micrometers. More preferably, the flow battery is capable of operatingat a current efficiency of greater than 99.5% when the separator has athickness of less than about 50 micrometers, a current efficiency ofgreater than 99% when the separator has a thickness of less than about25 micrometers, and a current efficiency of greater than 98% when theseparator has a thickness of less than about 10 micrometers. Suitableseparators include those separators in which the flow battery is capableof operating at a voltage efficiency of greater than 60% with a currentdensity of 100 mA/cm². More preferably, suitable separators includethose separators in which the flow battery is capable of operating at avoltage efficiency of greater than 70%, greater than 80% or even greaterthan 90%.

Separators are generally categorized as either solid or porous. Solidmembranes typically comprise an ion-exchange membrane, wherein anionomer facilitates mobile ion transport through the body of thepolymer. It is suitable for the ionomer to have an ionomer mass contenton an areal basis of less than 2×10⁻³ g ionomer/cm². The facility withwhich ions conduct through the membrane can be characterized by aresistance, typically an area resistance in units of Ωcm². The arearesistance is a function of inherent membrane conductivity and themembrane thickness. Thin membranes are desirable to reduceinefficiencies incurred by ion conduction and therefore can serve toincrease voltage efficiency of the energy storage device. Activematerial crossover rates are also a function of membrane thickness, andtypically decrease with increasing membrane thickness. Crossoverrepresents a current efficiency loss that is generally balanced with thevoltage efficiency gains by utilizing a thin membrane.

The ability to measure the permeability of the charged redox activematerial through a given membrane in the absence of charge passingthrough the flow battery may be conveniently measured, for example, by(a) providing a two chamber cell, each chamber separated by the suitablemembrane or separator of a specified thickness, filling each chamberwith an electrolyte composition, the first electrolyte containing themetal ligand coordination complex of interest and the second devoid ofsuch complex; (b) maintaining the chamber at a constant temperature(e.g., in a range of from about 20° C. to about 85° C.) for a timesuitable for the system (e.g., in a range of from about 1 hour to about120 hours); (c) measuring and quantifying the amount of metal ligandcoordination complex which has passed through the membrane from thefirst to the second electrolyte (using, for example, atomic absorptionor UV-Vis spectroscopy, ion chromatography, or other suitable method);and then (d) calculating the amount of metal ligand coordination complexwhich has passed through the membrane area over that period of time. Byvarying the time and temperature of such tests, as well as the membranethicknesses, it is also possible to calculate the thermodynamicparameter associated with this particular system, and predict longevityof the system.

It is preferred that the first electrolyte substantially comprises thefirst active material and be substantially free of the second activematerial, and that second electrolyte substantially comprises the secondactive material and be substantially free of the first active material,but over time, the concentration of the first active material mayincrease in the second electrolyte and the concentration of the secondactive material may increase in the first electrolyte. Suitable flowbatteries include batteries where the first active material is presentin the second electrolyte at a concentration no greater than about 0.05M. Conversely, the second active material may be present in the firstelectrolyte at a concentration no greater than about 0.05 M. Preferably,the concentration of the first active material is present in the secondelectrolyte and the second active material is present in the firstelectrolyte at a concentration of not greater than about 0.01 M, or nogreater than about 0.001 M or substantially free of the activematerials. The diffusion rate of the either the first or second activematerial through the membrane should be less than about 1×10⁻⁵ mol cm⁻²day⁻¹, less than about 1×10⁻⁶ mol cm⁻² day⁻¹, less than about 1×10⁻² molcm⁻² day⁻¹, less than about 1×10⁻⁹ mol cm⁻² day⁻¹, less than about1×10⁻¹¹ mol cm⁻² day⁻¹, less than about 1×10⁻¹³ mol cm⁻² day⁻¹, or lessthan about 1×10⁻¹⁵ mol cm⁻² day⁻¹.

Porous membranes are non-conductive membranes which allow chargetransfer between two electrodes via open channels filled with conductiveelectrolyte. Porous membranes are permeable to liquid or gaseouschemicals. This permeability increases the probability of chemicalspassing through porous membrane from one electrode to another causingcross-contamination and/or reduction in cell energy efficiency. Thedegree of this cross-contamination depends on, among other features, thesize (the effective diameter and channel length), and character(hydrophobicity/hydrophilicity) of the pores, the nature of theelectrolyte, and the degree of wetting between the pores and theelectrolyte.

The pore size distribution is generally sufficient to substantiallyprevent the crossover of active materials between the two electrolytesolutions. Suitable porous membranes will have an average sizedistribution of between about 0.001 nm and 20 micrometers. Preferably,the average size distribution should be between about 0.001 nm and 100nm. The size distribution of the pores in a porous membrane can besubstantial. In other words, a porous membrane may contain a pluralityof pores with a very small diameter (approximately less than 1 nm) andmay contain a plurality of pores with a very large diameter(approximately greater than 10 micrometers). The larger pore sizes canlead to a higher amount of active material crossover. The ability for aporous membrane to substantially prevent the crossover of activematerials will depend on the relative difference in size between theaverage pore size and the active material. For example, when the activematerial is a metal center in the form of a metal-ligand complex, theaverage diameter of the metal ligand complex is about 50% greater thanthe average pore size of the porous membrane. On the other hand, if theporous membrane has substantially uniform pore sizes, it is preferredthat the average diameter of the metal ligand complex be about 20%larger than the average pore size of the porous membrane. Likewise, theaverage diameter of a metal ligand complex is increased when themetal-ligand complex is further coordinated with at least one watermolecule. The diameter of the metal-ligand complex coordinated with atleast one water molecule is generally considered to be the hydrodynamicdiameter. In such a situation, the hydrodynamic diameter is generally atleast about 35% greater than the average pore size. When the averagepore size is substantially uniform, the hydrodynamic radius should beabout 10% greater than the average pore size. One of ordinary skill inthe art will understand the term “substantially uniform.”

Suitable ion-exchange separators may also comprise membranes, which aresometimes referred to as polymer electrolyte membrane (PEM) or ionconductive membrane (ICM). Suitable membranes may comprise any suitablepolymer, typically an ion exchange resin, for example comprising apolymeric anion or cation exchange membrane, or combination thereof. Themobile phase of such a membrane may comprise, and/or is responsible forthe primary or preferential transport (during operation of the battery)of at least one mono-, di-, tri-, or higher valent cation and/or mono-,di-, tri-, or higher valent anion, other than protons or hydroxide ions.

Suitable solid cationic exchange polymers include use of one or more ofthe following polymers: cross-linked halogenated alkylated compound witha polyamine, a cross-linked aromatic polysulfone type polymer with apolyamine, perfluorinated hydrocarbon sulfonate ionomers, sulfonatedpoly ether ether ketone (sPEEK), sulfonated poly(phthalazinone etherketone), sulfonated phenolphthalein poly(ether sulfone), sulfonatedpolyimides, sulfonated polyphosphazene, sulfonated polybenzimidazole,aromatic polymers containing a sulfonic acid group, sulfonatedperfluorinated polymer, fluorinated ionomers with sulfonate groups,carboxylate groups, phosphate groups, boronate acid groups, polyaromaticethers with sulfonate or carboxylate groups, poly(4-vinyl pyridine,poly(2-vinyl pyridine), poly(styrene-b-2-vinylpyridine), poly(vinylpyrrolidine), poly(l-methyl-4-vinylpyridine),poly[(2,2′-m-phenylene)-5,5′-bibenzimidazole][poly(2,2′-(m-phenylene)-5,5-′-bibenzimidazole],poly(2,5-benzimidazole), polyacrylate, polymethacrylate or combinationsthereof. Suitable solid anionic exchange membranes include the use ofone or more of the following polymers: polydiaryl dimethyl ammonium,poly(methacaryloxyloxyethyl triethylammonium), poly(diallylammonium), orcombinations thereof.

Additionally, substantially non-fluorinated membranes that are modifiedwith sulfonic acid groups (or cation exchanged sulfonate groups) mayalso be used. Such membranes include those with substantially aromaticbackbones, e.g., poly-styrene, polyphenylene, bi-phenyl sulfone (BPSH),or thermoplastics such as polyetherketones or polyethersulfones.

Other examples of ion-exchange membranes comprise Nafion™ (112, 117, HP,XL, NR-212, or U5), Gore Select membranes, Flemion™, and Selemion™.

Battery-separator style porous membranes, may also be used. Because theycontain no inherent ionic conduction capability, such membranes aretypically impregnated with additives in order to function. Thesemembranes are typically comprised of a mixture of a polymer, andinorganic filler, and open porosity. Suitable polymers include thosechemically compatible with the electrolytes of the presently describedsystems, including high density polyethylene, polypropylene,polyvinylidene difluoride (PVDF), or polytetrafluoroethylene (PTFE).Suitable inorganic fillers include silicon carbide matrix material,titanium dioxide, silicon dioxide, zinc phosphide, and ceria and thestructures may be supported internally with a substantiallynon-ionomeric structure, including mesh structures such as are known forthis purpose in the art.

Membranes may also be comprised of polyesters,poly(ether-ketone-ether-ketone-ketone), poly(vinyl chloride), vinylpolymers, substituted vinyl polymers, alone or in combination of anypreviously described polymer.

Membranes may also comprise reinforcement materials for greaterstability. Suitable reinforcement materials include nylon, cotton,polyesters, crystalline silica, crystalline titania, amorphous silica,amorphous titania, rubber, asbestos wood or combination thereof. Thevolume percent of a reinforcement material may be determined at a givenmembrane thickness by the following example. The reinforcement materialpercent is determined from Equation (11):

Desired thickness=starting membrane thickness/(1−reinforcement vol%)  (11)

For example, the membrane should contain about 33% reinforcementmaterial by volume starting with a membrane of 10 micrometers with adesired thickness of 15 micrometers.

Suitable membranes also comprise continuous membranes. The continuousmembranes comprise at least a material in a continuous or discontinuousstructure and a filler material that is a continuous or discontinuousstructure. Suitable materials in a continuous or discontinuous structurecomprises one or more of polyethylene, polypropylene,poly(tetrafluoroethylene), poly(vinyl chloride), or a combinationthereof. Suitable filler material in a continuous or discontinuousstructure comprises one or more of nonwoven fibers or naturallyoccurring substances. Suitable nonwoven fibers comprises one or more ofnylon, cotton, polyesters, crystalline silica, amorphous silica,amorphous titania, crystalline titania or a combination thereof.Suitable naturally occurring substances comprise one or more of rubber,asbestos, wood or combination thereof. The continuous membrane may alsobe porous. Suitable porosity is in the range of about 5 to about 75%volume fraction.

Suitable separators may also have a plurality of layers. For instance, asuitable separator comprises a layer capable of ionic conduction and atleast one layer capable of selective ion transport. The layer capable ofionic conduction comprises at least one of either the first electrolyteor the second electrolyte imbibed in to the separator. The electrolytesolution, for example potassium chloride (KCl), becomes imbibed into theseparator and does not substantially seep out from the polymer matrix.The desired areal resistance range for the imbibed separator isdetermined by Equation (12):

R _(total)[ohm-cm² ]=K _(membrane)/10E-6 m+(porosity_(sep)̂1.5*K_(electrolyte))/thickness_(sep)  (12)

where R is the resistance, K_(membrane) is the conductivity of themembrane, K_(electrolyte) is the conductivity of the electrolyte,porosity_(sep) is the porosity of the separator and thickness_(sep) isthe thickness of the separator. Any inert electrolyte, such as NaCl, KClor the like, is suitable. One of ordinary skill in the art willappreciate suitable inert electrolytes suitable for this purpose. Thelayer capable of selective ion transport comprises any of the abovementioned solid cationic polymers. Other layers are envisioned withinthe scope of this invention that may enhance or reduce properties suchas conduction, strength, thickness, selectivity, permeability, or thelike.

Suitable separators include those separators capable of having aselectivity of at least between about 50 to about 300 for at least onemobile ion over the any present active material. Suitable separators arecapable of having a selectivity of at least between about 100 to about200, and between at least about 50 to about 100 for at least one mobileion over any present active material.

In either an on-load or off-load condition, there may exist asignificant difference in the concentration of active material speciesin the positive and negative electrolytes in a region of a cell. Despitethe presence of the separator, there always exists some finite flux ofthese species across it due to these concentrations differences sinceall separators exhibit some permeability. When these species crossover,a loss of energy efficiency occurs since charged species areself-discharging through direct interaction, but also the potential forelectrolyte regeneration exists if the battery employs different activematerial compounds. It is of interest to develop a flow batterychemistry and cell configuration whereby the losses due to diffusivecrossover of active materials from either electrolyte to the other donot, in total, exceed 2% of the current in an on-load condition incharge or discharge mode, preferably <(“less than”) 1%, and mostpreferably <<(“much smaller than”) 1% for the reasons provided above.

Suitable separators include those separators where the separator iscapable of excluding at least about 98% of at least one of the activematerials. Preferably, the separator is capable of excluding at leastabout 99.0% of at least one of the active materials, and at least about99.5% of the active materials.

When constructing practical flow battery cells, the electrodes mayslightly permeate the separator and result in electrical shorting in aregion of a cell. This facilitates the direct exchange of electronsacross those shorts, which represents another form of self-dischargeleading to current efficiency loss. Flow battery design generallyincludes a desired combination of separator mechanical properties (i.e.,strength), diffusion media properties, cell geometry, and cellmechanical loading. It is of interest to develop a flow batterychemistry and cell configuration whereby the losses due to electricalshorts, in total, exceed 2% of the current in an on-load condition incharge or discharge mode.

Suitable separators are separators which are characterized as having aconductivity of about 0.01 to about 0.06 S/cm for Li⁺, Na⁺, and/or K⁺and a conductivity of about less than 0.03 S/cm for Cl⁻, Br⁻, I⁻, and/orOH⁻.

In an on-load condition during charge or discharge in a region of a flowbattery cell, ionic current must flow across the separator during thecourse of the operation. It is desired that most of the ionic currentdemand be carried by mobile ions provided by supporting species in theelectrolyte. However, if the active materials are ionically charged,they may participate in carrying some portion of the ionic currentdemand, which depends on their transference. Significant transference ofactive materials occurs during the course of charge or dischargerepresents yet another form of self-discharge leading to currentefficiency losses. It is of interest to develop a flow battery chemistryand cell configuration whereby the transference of active materials fromeither electrolyte to the other do not, in total, exceed 2% of thecurrent in an on-load condition in charge or discharge mode, preferably<1%, and most preferably <<1% for the reasons provided above.

A portion of the cell geometry may contain an active area. It isdesirable for at least a portion of the active area to be comprised ofchannels. The channels are largely open to the flow of electrolytes andportions of an electrically conductive plate material that electricallyconnects the electrodes either directly or through diffusion media.Conversely, it is suitable for the active area to be substantiallyformed of a region that is permeable to the flow of either the firstelectrolyte or second electrolyte, and whose volume is comprisedpartially of a high surface area, electrically conducting media.

A suitable flow battery is capable of a cell mechanical loading beingable to withstand a mechanical load in the range of about 1 to about1000 psig. Preferably, the flow battery is capable of withstanding amechanical load of in the range of about 3 to about 500 psig, and morepreferably between about 5 to about 100 psig.

In an on-load condition during charge or discharge in a flow batterycell, there may exist the potential for the current to be consumed inundesirable side reactions. Such side reactions include corrosion ofcell materials, decomposition of the active material structure, ordecomposition of the electrolyte. This is especially true wheresignificant non-uniformities in concentration, voltage, or currentdensity exist across the cell area. It is of interest to develop a flowbattery chemistry and cell configuration whereby the current lost inparasitic reactions does not, in total, exceed 4% of the current in anon-load condition in charge or discharge mode, preferably <2%, and mostpreferably <1% for the reasons provided above.

Flow batteries are comprised of cells stacked in a bipolar configurationwhereby the active materials are fed to either or both the positive andnegative electrolyte chambers through common manifolds. Since theseelectrolytes are ionically conductive, their presence in a commonmanifold results in positive ionic current being driven from cellstowards the positive end of the stack to those towards the negative end.This process will occur in both the positive and negative electrolytemanifolds, and will represent yet another form of self-discharge andcurrent efficiency loss. It is of interest to develop a flow batterychemistry and cell/stack configuration whereby the current lossesrepresented by shunt currents do not, in total, exceed 5% of the currentin an on-load condition in charge or discharge mode, preferably <3%, andmost preferably <2% for the reasons provided above.

The open circuit potential (OCV) of an electrochemical cell is arelevant operating characteristic of electrochemical energy storagesystems. In certain embodiments, the OCV may be comparatively large(e.g. greater than about 1 V). Such comparatively large open circuitpotentials are known to enable high cell voltage efficiencies, highAC-AC conversion efficiencies, high energy storage densities, and lowsystem costs. Traditional flow batteries with aqueous electrolytes andsoluble active materials may operate with an OCV less than about 1.2 V.An electrochemical cell according to the present disclosure is suitablycharacterized by an open circuit potential of at least about 1.4 V.

The novel electrolytes of the present invention may provide the opencircuit voltages (OCVs) of the flow battery of at least about 0.8 volts,at least about 0.9 V, at least about 1.0 V, at least about 1.1 V, leastabout 1.2 volts, at least about 1.3 V, at least about 1.4 V, at leastabout 1.5 V, at least about 1.6 V, at least about 1.7 V, at least about1.8 V, at least about 1.9 V, or at least about 2 V. As described above,higher open circuit voltages are associated with higher power densities.

The present disclosure presents exemplary cyclic voltammetry data forseveral metal ligand coordination compound couples under a range ofconditions (see Tables 2 and 3). In considering these (or other) sets ofhalf-cell couples, certain embodiments provide that the cells comprisethose pairs of metal ligand coordination compounds whose couples providelarge open circuit potential, while capable of operating at potentialsthat are within the potentials associated with the generation ofhydrogen and oxygen derived from the electrolysis of water (i.e., so asto operate at potentials where the generation of a hydrogen or oxygenevolution current is minimized or avoided). In certain embodiments,these half-cell couples are chosen to provide large open circuitvoltages while operating at or below a half-cell potential of 0 V at thenegative electrode and at or above a half-cell potential of 1.23 V atthe positive electrode, where the half-cell potentials are with respectto a reversible hydrogen electrode. Through judicious choice ofelectrode materials which exhibit poor catalytic activity, e.g., anallotrope of carbon or a metal oxide, it is possible to provide systemshaving large overpotentials, so as to drive the OCV to values higherthan the thermodynamic limit of 1.23 V without hydrogen or oxygenevolution. For example, experiments show (and as reflected in Table 3below) the Ti^(4+/3+)(cat)₃ ^(2−/3−) and Al(cit)₂(cat)^(2−/3−) pair ofcouples can exhibit an OCV of 1.73 V using carbon electrodes

Systems and methods according to the present disclosure may exhibit aparticular current density at a given round trip voltage efficiency.Methods for determining current density at a given round trip voltageefficiency are known to those skilled in the art of electrochemistry andelectrochemical energy storage.

To serve as a metric for electrochemical cell performance, a specifiedcurrent density is generally linked to a measured voltage efficiency.Higher current densities for a given round trip voltage efficiencyenable lower cost electrochemical cells and cell stacks. In certainembodiments, it is desired to operate a flow battery with a currentdensity greater than about 50 mA/cm² at V_(EFF,RT) greater than about50%. In other embodiments, the current density will be greater thanabout 50 mA/cm² at V_(EFF,RT) greater than about 60%, greater than about75%, greater than about 85%, greater than about 90%. In otherembodiments, the current density will be greater than 100 mA/cm² atV_(EFF,RT) greater than about 50%, greater than about 60%, greater thanabout 75%, greater than about 85%, greater than about 90% and the like.In other embodiments, the current density will be greater than 200mA/cm² at V_(EFF,RT) greater than about 50%, greater than about 60%,greater than about 75%, greater than about 85%, greater than about 90%,and above.

Electrolytes that include an organic active material, either in theabsence or presence of metal coordination, are considered suitable forone or both half-cells of the disclosed systems and methods. Suitableorganic active materials include carbon, aromatic hydrocarbons,including quinones, hydroquinones, viologens, pyridinium, pyridine,acridinium, catechols, other polycyclic aromatic hydrocarbons, and thelike. Suitable organic active materials may also include sulfur,including thiol, sulfide, and disulfide moieties. Suitable organicactive materials may be soluble in water in concentrations greater than0.1 M, greater than 0.5 M, greater than 1 M, greater than 1.5 M, greaterthan 2 M, and above.

The disclosed systems and methods may also be characterized in terms oftheir half-cell potentials. Both the negative and positive electrode mayexhibit a half-cell potential. An electrochemical cell according to thepresent disclosure may, in some embodiments, have a half-cell potentialfor the negative electrode less than about 0.5 V vs. RHE, less thanabout 0.2 V vs. RHE, less than about 0.1 V vs. RHE, less than about 0.0V vs. RHE, less than about −0.1 V vs. RHE, less than about −0.2 V vs.RHE, less than about −0.3 V vs. RHE, less than about −0.5 V vs. RHE. Anelectrochemical cell according to the present disclosure may, in someembodiments, have a half-cell potential for the positive electrodegreater than about 0.5 V vs. RHE, greater than about 0.7 V vs. RHE,greater than about 0.85 V vs. RHE, greater than about 1.0V vs. RHE,greater than about 1.1V vs. RHE, greater than about 1.2V vs. RHE,greater than about 1.3 V vs. RHE, greater than about 1.4 V vs. RHE andthe like.

The disclosed systems and methods may also be characterized in terms oftheir energy density, as defined above. Flow batteries of the presentdisclosure may operate with an energy density in excess of about 5 Wh/L,of about 10 Wh/L, excess of about 15 Wh/L, of about 20 Wh/L, excess ofabout 25 Wh/L, of about 30 Wh/L, excess of about 35 Wh/L, of about 40Wh/L, or between about 5 Wh/L and about 15 Wh/L, between about 10 Wh/Land about 20 Wh/L, between about 20 Wh/L and about 30 Wh/L, betweenabout 30 and about 40 Wh/L, between about 25 Wh/L and about 45 Wh/L, andabove 45 Wh/L.

Example 1 General

The following Examples are provided to illustrate some of the conceptsdescribed within this disclosure. While each Example is considered toprovide specific individual embodiments of composition, methods ofpreparation and use, none of the Examples should be considered to limitthe more general embodiments described herein.

Example 1.1 Materials

Sodium hexacyanoferrate(II) decahydrate 99%, Na₄Fe(CN)₆10H_(2O);potassium hexacyanoferrate(II) trihydrate 98+%, K₄Fe(CN)₆3H₂O; andpotassium hexacyanoferrate(III) ACS 99.0% min, K₃Fe(CN)₆ were purchasedfrom Alfa Aesar (Ward Hill, Mass.) and used without additionalpurification. Potassium hexacyanochromate(III), K₃[Cr(CN)₆] andpotassium hexaycyanomanganate(III), K₃[Mn(CN)₆] were purchased fromSigma-Aldrich (St. Louis, Mo.) and used without additional purification.

Disodium titanium(IV) triscatecholate, Na₂Ti(catecholate)₃ wassynthesized by a modification of a procedure described by Davies, seeDavies, J. A.; Dutramez, S. J. Am. Ceram. Soc. 1990, 73. 2570-2572, fromtitanium(IV) oxysulfate and pyrocatechol. Sodium hydroxide was used inplace of ammonium hydroxide to obtain the sodium salt. Sodium potassiumtitanium(IV) trispyrogallate, NaKTi(pyrogallate)₃ was made analogously,first as the ammonium salt, (NH₄)Ti(pyrogallate)₃, and subsequentlyconverted to the sodium potassium salt by heating in a mixture ofaqueous sodium hydroxide and aqueous potassium hydroxide.

The mixed ligand titanium complexes sodium potassium titanium(IV)biscatecholate monopyrogallate, sodium potassium titanium(IV)biscatecholate monolactate, sodium potassium titanium (IV)biscatecholate monogluconate, sodium potassium titanium(IV)biscatecholate monoascorbate, and sodium potassium titanium(IV) biscatecholate monocitrate were made from a titanium catecholate dimer,Na₂K₂[TiO(catecholate)]₂. For the synthesis of the tetrapotassium saltsee Borgias, B. A.; Cooper, S. R.; Koh, Y. B.; Raymond, K. N. Inorg.Chem. 1984, 23, 1009-1016. A one-to-one mixture of titanium dimer withthe desired chelate (pyrogallol, lactic acid, gluconic acid, ascorbicacid, or citric acid) gave the mixed ligand species. Sodium potassiumtitanium(IV) monocatecholate monopyrogallate monolactate was made in asimilar fashion by addition of both pyrogallol and lactic acid to thecatecholate containing dimer. Mixed ligand analogs of the Al, Cr, and Fecompounds may be prepared by similar reaction schemes. The structures ofseveral of the titanium compounds were confirmed by mass spectroscopy(see Table 1).

TABLE 1 Mass spectroscopy for selected compounds Mass (m/z) Calc'd/Obs'dTi(catecholate)₃ ²⁻ 186.0080/186.0 Ti(pyrogallate)₃ ²⁻ 210.0038/210.0Ti(catecholate)₂(pyrogallate)²⁻ 194.0055/194.0Ti(catecholate)₂(ascorbate)²⁻ 219.0057/219.0Ti(catecholate)₂(gluconate)²⁻ 229.0188/229.0 Ti(catecholate)₂(lactate)²⁻176.0055/176.0 * Mass spectrometry data were obtained on an Agilent6150B single quadrupole LC/MS in the negative ion mode with electrosprayionization (ESI). Aqueous solution samples of the metal ligand complexwere diluted in methanol and introduced to the mass spectrometer ionizerby direct injection using a syringe pump. The reported m/z peaks in eachcase are for the dianions, z = −2.

Sodium potassium iron(III) triscatecholate, Na₁₅K₁₅Fe(catecholate)₃ wasprepared according to the procedure outline by Raymond et. al., seeRaymond, K. N.; Isied, S. S., Brown, L. D.; Fronczek, F. R.; Nibert, J.H. J. Am. Chem. Soc. 1976, 98, 1767-1774. The only modification was theuse of a mixture of sodium hydroxide and potassium hydroxide as theexcess base in place of potassium hydroxide.

Sodium titanium(IV) triscitrate, Na₄Ti(citrate)₃, was synthesized byanalogy to the method used for sodium titanium(IV) tricatecholatedescribed above except using citric acid in place of catechol. Thesestarting materials were obtained from Alfa Aesar (Ward Hill, Mass.),were of reagent grade or better, and were used as received.

Sodium aluminum(III) biscitrate monocatecholate,Al(citrate)₂(catecholate), was synthesized in analogy to the method usedfor sodium titanium(IV) tricatecholate described above except using twoequivalents of citric acid and one equivalent of catechol to a solutionof aluminum(III) sulfate. These starting materials were obtained fromAlfa Aesar (Ward Hill, Mass.), were of reagent grade or better, and wereused as received.

Example 1.2 Cyclic Voltammetry

Cyclic voltammetry data was recorded using a 760c potentiostat (CHInstruments, Austin, Tex.) with iR correction. Tests were conductedusing glassy carbon working electrodes (Bioanalytical Systems, Inc.,West Lafayette, Ind.), Ag/AgCl reference electrodes (BioanalyticalSystems, Inc. West Lafayette, Ind.) and platinum wire counter electrodes(Alfa Aesar, Ward Hill, Mass.). Working electrodes were polishedaccording to the supplier's instructions before each experiment.Reference electrodes were calibrated against a “master” Ag/AgClelectrode known to have a potential of +0.210 V vs. NHE as known bythose skilled in the art of electrochemistry. Solutions were spargedwith argon for at least 5 minutes before each experiment. Allexperiments were performed at ambient temperatures (17-22° C.). Nosupporting electrolytes were added unless otherwise specified. All datawere collected at a scan rate of 100 mV/s unless otherwise specified.Under these conditions, hydrogen evolution became significant atpotentials more negative than −0.80 V vs. RHE and oxygen evolutionbecame significant at potentials more positive than +2.20 V vs. RHE.

Example 1.3 Experimental Procedure for a 5 cm² Active Area Flow Battery

Cell hardware designed for 5 cm² active area and modified for acid flowwas obtained from Fuel Cell Technologies (Albuquerque, N. Mex.). Carbonfelt, nominally 3 mm thick, was obtained from Alfa Aesar (Ward Hill,Mass.). Felts were dip-coated with a suspension of Vulcan XC-72 carbon(Cabot Corp., Boston, Mass.) and NAFION™ (Ion-Power, New Castle, Del.)and air-dried before use. NAFION™ HP, XL, or NR-212 cation exchangemembranes were obtained from Ion-Power. Viton™ gaskets were obtainedfrom McMaster Carr (Robinsville, N.J.) and were cut to allow for a 5 cm²active area with ˜1 cm² areas left above and below the felts forelectrolyte ingress and egress from the positive and negativecompartments of the cell. The cell was assembled using gaskets thatprovided a compression of ˜25% of the measured thickness of the felts.The membranes and electrodes were not pretreated before assembly. Theelectrolyte reservoirs were fashioned from Schedule 80 PVC piping withPVDF tubing and compression fittings. Masterflex™ L/S peristaltic pumps(Cole Parmer, Vernon Hills, Ill.) were used with Tygon™ tubing.Electrolytes were sparged with UHP argon through an oil-filled bubbleroutlet before electrochemical testing. An Arbin Instruments BT2000(College Station, Tex.) was used to test the electrochemicalperformance, and a Hioki 3561 Battery HiTESTER (Cranbury, N.J.) was usedto measure the AC resistance across the cell.

In a typical experiment, 50 mL each of electrolyte containing activematerial for the positive and negative electrode were loaded intoseparate reservoirs and sparged with argon for 20 minutes whilecirculating the electrolytes through the cell. The electrolytes werecharged to 40% SOC (calculated from the concentrations of the activematerials and the volumes of the electrolyte), the iV response of thecell was obtained, and then the electrolytes were cycled between 40 and60% SOC. An analog output from the Hioki battery tester was recorded tomonitor changes in the membrane and contact resistances.

Example 2

A redox flow battery cell was assembled according to the methodsdescribed in Example 1.3 using titanium tris-catecholate(Ti^(4+/3+)(cat)₃ ^(2−/3−)) and ferri/ferro-cyanide (Fe^(3+/2+)(CN)₆^(3−/4−)) metal ligand coordination compounds as active materials forthe negative and positive electrolytes, respectively. The activematerials were prepared at concentrations of 0.5 M in 0.5 M pH 11 Na₂SO₄supporting electrolyte (negolyte) or no supporting electrolyte(posolyte) and were flowed at 100 mL/min through the flow battery cellassembled using 5 cm² carbon felt electrodes and a NAFION™ cationselective membrane (50 μm thick) in Na form. The cell was initiallycharged from 0 to 50% state of charge before several charge/dischargecycles was collected by sweeping the cell current from open circuit to˜150 mA/cm² and monitoring the resulting cell potential, FIG. 2. At opencircuit, a cell potential of 1.63 V was observed as expected forequilibrium cell potential at 50% SOC based on the externally measuredE_(1/2) values for Ti^(4+/3+)(cat)₃ ^(2−/3−) and Fe^(3+/2+)(CN)₆^(3−/4−). Charge/discharge cycling revealed well behaved, reproduciblevoltage/current vs. time traces, demonstrating promising durability,FIG. 2. An RT voltage efficiency of 69% was measured for this system at150 mA/cm². Typical resistances measured by the Hioki Battery Tester forthe membrane and contact resistance component of cells built with NR212,XL, and HP membranes were 0.77, 0.60, and 0.5 .OMEGA.cm², respectively.

FIG. 3 displays the charge/discharge characteristics for a flow batteryof the present invention wherein the negative and positive activematerials comprise Ti^(4+/3+)(cat)₃ ^(2−/3−) and Fe^(3+/2+)(CN)₆^(3−/4−), respectively. The cell potential increases as the battery ischarged and decreases as the battery is discharged.

Example 3

A redox flow battery cell was assembled according to the methodsdescribed in Example 1.3 using titanium tris-catecholate(Ti^(4+/3+)(cat)₃ ^(2−/3−)) and ferri/ferro-cyanide (Fe^(3+/2+)(CN)₆^(3−/4−)) metal ligand coordination compounds as active materials forthe negative and positive electrolytes, respectively. In a typical cell,stable voltages were observed upon repeatedly charging to 60% SOC anddischarging to 40% SOC (see FIG. 4) when the discharge energy for eachcycle was 99.8% of the charge energy, indicative of 99.8% roundtripcurrent efficiency. This was achieved by using a constant currentdensity (e.g., 150 mA/cm²) for both charge and discharge but with adischarge time that was slightly shorter than (i.e., 99.8% of) thecharge time. Under these conditions, the open circuit voltages at 40 and60% SOC were stable for extended periods of time.

Crossover flux data were obtained by measuring the concentrations of Feand Ti in each electrolyte at the beginning and end of a suitablylengthy battery test, typically one to two weeks in duration for amembrane area of 7 cm². The concentrations were determined byInductively Coupled Plasma-Mass Spectrometry (ICP-MS) experimentsperformed by Evans Analytical Group, Syracuse, N.Y. The moles of Fe inthe Ti-containing electrolyte before the test were subtracted from thenumber of moles in the same electrolyte at the end of the test. This wasconverted to a flux by dividing the moles by the membrane area and thetest duration.

Typical fluxes for boiled DuPont Nafion™ NR212 (50 μm thick) were5.0×10⁻⁸ mol cm⁻² day⁻¹ for ferri/ferrocyanide and 6.5×10⁻⁸ mol cm⁻²day⁻¹ for titanium triscatecholate. For unboiled DuPont Nafion™ HP (20μm thick), the measured fluxes were 1.1×10⁻⁵ and 3.3×10⁻⁶ mol cm⁻² day⁻¹for the above iron and titanium complexes, respectively. It should benoted that these fluxes are substantially lower than 1% of the totalcurrent (and thus the total moles of ions passed across the membrane)during this time. For example, in the NR212 test above, 6.4×10⁻² mol oftotal ions were passed over 6.8 days of operation at 100 mA/cm²,approximately 6 orders of magnitude larger than the amount of activematerial ion crossover.

Example 4

A redox flow battery cell was assembled according to the general methodsdescribed in Example 1.3, again using titanium bis-catecholatemono-pyrogallate (Ti^(4+/3+)(cat)₂(gal)^(2−/3−)) and ferri/ferro-cyanide(Fe^(3+/2+)(CN)₆ ^(3−/4−)) metal ligand coordination compounds as activematerials for the negative and positive electrolytes, respectively. Inthis example the carbon felt electrodes were replaced with TORAY carbonpaper electrodes that were catalyzed with Vulcan carbon and NAFION™ in amanner similar to that of Example 2. Additionally, flow fields of the“interdigitated” type were employed. The active material solutionconcentrations were increased to 1.5 M and the cell performance wasevaluated by monitoring the cell potential on both charge and dischargecycles as a function of current density. As can be seen in FIG. 5, thecell maintains round trip voltage efficiencies of 84%, 79%, and 73% atcurrent densities of 150, 200, and 250 mA/cm², respectively. In thisconfiguration the flow battery active materials exhibited an energydensity of 32.79 Wh/L.

The results of analogous experiments using Ti^(4+/3+)(cat)₃ ^(2−/3−) andFe^(3+/2+)(CN)₆ ^(3−/4−) are shown in FIG. 6.

Example 5 Cyclic Voltammetry Data

The following experiments provide information as to the nature of thehalf-cell performance for the indicated materials. As described above,certain embodiments of the present invention include those flowbatteries comprising these, or analogous, materials which would providefull cell performance reflective of the reported half-cell performance,and such embodiments are considered within the scope of the presentinvention.

TABLE 2 Exemplary electrochemical couples described herein SolubilityCharge E_(1/2), V (Molar), Density Couple vs. RHE pH FIG. 25° C. (Ah/L)Al(citrate)₂(catecholate)^(2−/3−) 1.25 11.5 8 0.5 13.4Fe(catecholate)^(2−/3−) −0.50 11 10 1.5 40.2 Ti(catecholate)^(2−/3−)−0.45 11 15 1.0 26.8 Ti(pyrogallate)^(2−/3−) −0.55 9.8 9 1.6 42.9Ti(catecholate)₂(pyrogallate)^(2−/3−) −0.50 11 11 1.5 40.2Ti(catecholate)₂(ascorbate)^(2−/3−) −0.55 10 14 1.5 40.2Ti(catecholate)₂(gluconate)^(2−/3−) −0.60 9 13 1.5 40.2Ti(catecholate)₂(lactate)^(2−/3−) −0.49 9 12 1.5 40.2Ti(catecholate)(pyrogallate)(lactate)^(2−/3−) −0.70 8.5 16 1.5 40.2Ti(citrate)₃ ^(2−/3−) −0.04 5 17 2.0 53.6 Fe(CN)₆ ^(3−/4−) 1.18 11 181.5 40.2 Cr(CN)₆ ^(3−/4−) −0.60 9 19 1.5 40.2 Mn(CN)₆ ^(3−/4−) −0.60 920 1.5 40.2

TABLE 3 Calculated OCVs and theoretical energy density (Wh/L) forvarious electrolyte couple pairs calculated from data in Table 2 Fe(CN)₆^(3−/4−) Al(cit)₂(cat)^(2−/3−) Energy Energy OCV Density OCV DensityCouple (V) (Wh/L) (V) (Wh/L) Mn(CN)₆ ^(3−/4−) 1.78 35.8 1.85 12.4Fe(catecholate)^(2−/3−) 1.68 33.8 1.75 11.7 Ti(catecholate)^(2−/3−) 1.6321.8 1.70 11.4 Ti(pyrogallate)^(2−/3−) 1.73 34.8 1.80 12.1Ti(catecholate)₂(pyrogallate)^(2−/3−) 1.68 33.8 1.75 11.7Ti(catecholate)₂(ascorbate)^(2−/3−) 1.73 34.8 1.80 12.1Ti(catecholate)₂(gluconate)^(2−/3−) 1.78 35.8 1.85 12.4Ti(catecholate)₂(lactate)^(2−/3−) 1.67 33.6 1.74 11.7Ti(catecholate)(pyrogallate)(lactate)^(2−/3−) 1.73 34.8 1.80 12.1Ti(citrate)₃ ^(2−/3−) 1.22 24.5 1.29 8.6

Example 5.1

Using an Al(cit)₂(cat)^(2−/3−) couple (E_(1/2)=˜1.25 V vs. RHE) as ademonstrative case, a high potential was observed with well-behavedelectrochemical signatures at glassy carbon electrodes, FIG. 8. Whencoupled with the Ti⁴⁺(cat)₃ ²⁻ complex described above these pairs maygive aqueous battery pairs with OCVs of ˜1.7-1.9 V.

Examples 5.2 and 5.3

FIG. 9 (for titanium tris-pyrogallate) and FIG. 10 (for irontris-catecholate) illustrate the CV curves resulting from the use ofcatecholate-like ligands over a range of low and negative operatingpotentials, under conditions described above, showing the goodelectrochemical reversibility of these systems under these conditions.

Examples 5.4 through 5.10

FIG. 11 (NaK[Ti(catecholate)₂(pyrogallate)]), FIG. 12(NaK[Ti(catecholate)₂(lactate)]), FIG. 13(NaK[Ti(catecholate)₂(gluconate)]), FIG. 14(NaK[Ti(catecholate)₂(ascorbate)]), FIG. 15 (Na₂[Ti(catecholate)₃]),FIG. 16 (NaK[Ti(catecholate)(pyrogallate)(lactate)]), and FIG. 17(Na₄[Ti(citrate)₃]) illustrate the CV curves resulting from the use ofseveral mixed ligand or tris-citrate systems over a range of low andnegative operating potentials, under conditions described above, showingthe good electrochemical reversibility of these systems under theseconditions.

Example 5.11 Ferrocyanide Samples

Solid Na₄Fe(CN)₆.10H₂O (33.89 g, 0.070 mol) and K₄Fe(CN)₆.3H₂O (29.57 g,0.070 mol) were stirred in 80 mL deionized water. To dissolve thesolids, sufficient water was then slowly added to provide a samplecontaining ca. 1.5 M of Fe(CN)₆ ⁴⁻. This solubility was unexpected giventhat the solubilities of Na₄Fe(CN)₆.10H₂O and K₄Fe(CN)₆.3H₂O are eachknown in the art to be less than 0.7 M at the same ambient temperatures.

The 1.5 M [Fe(CN)₆]⁴⁻ solution was interrogated by cyclic voltammetry,using a glassy carbon working electrode. FIG. 18. In these experiments,sufficient solid sodium potassium hydrogen phosphate, NaOH, and KOH wasadded to the 1.4 M [Fe(CN)₆]⁴⁻ solution to yield a working solutionhaving a pH of 11.1 (ratio N⁺/K⁺ about.1) and containing 1.4 M[Fe(CN)₆]⁴⁻ and 0.1 M phosphate.

Examples 5.12 and 5.13

FIG. 19 (K₃[Cr(CN)₆]) and FIG. 20 (K₃[Mn(CN)₆]) illustrate the CV curvesresulting from the use of two other hexacyanide systems over a range oflow and negative operating potentials, under conditions described above,showing the good electrochemical reversibility of these systems underthese conditions.

Many of the embodiments thus far have been described in terms of flowbatteries in which at least one metal ligand coordination compounds isdescribed by the formula M(L1)_(3-x-y)(L2)_(x)(L3)_(y) ^(m). It shouldbe appreciated, however, that other embodiments include those where thehexacyanide compounds described herein may provide the basis of both ofthe positive and negative electrolytes. From FIG. 20, for example, itshould be apparent that the [Mn(CN)₆]^(3−/4−) and [Mn(CN)₆]^(4−/5−)couples, in addition to providing the basis of either positive ornegative electrolytes, in combination with other complementaryelectrolytes described herein as M(L1)_(3-x-y)(L2)_(x)(L3)_(y) ^(m), mayalso provide the basis for both the positive and negative electrolytesin a flow battery system. Similarly, independent embodiments alsoinclude those where the positive electrolyte comprises [Fe(CN)₆]^(3−/4−)and the negative electrolyte comprises [Cr(CN)₆]^(3−/4−) or[Mn(CN)₆]^(3−/4−).

As those skilled in the art will appreciate, numerous modifications andvariations of the present invention are possible in light of theseteachings, and all such are contemplated hereby. For example, inaddition to the embodiments described herein, the present inventioncontemplates and claims those inventions resulting from the combinationof features of the invention cited herein and those of the cited priorart references which complement the features of the present invention.Similarly, it will be appreciated that any described material, feature,or article may be used in combination with any other material, feature,or article, and such combinations are considered within the scope ofthis invention.

The disclosures of each patent, patent application, and publicationcited or described in this document are hereby incorporated herein byreference, each in its entirety.

What is claimed is the following:
 1. A composition comprising: atitanium complex containing at least one catecholate ligand; and sodiumand potassium counterions forming a salt of the titanium complex.
 2. Thecomposition of claim 1, wherein the titanium complex contains at leastone unsubstituted catecholate ligand and at least one substitutedcatecholate ligand.
 3. The composition of claim 2, wherein the at leastone substituted catecholate ligand comprises a sulfonated catecholateligand.
 4. The composition of claim 2, wherein the at least onesubstituted catecholate ligand comprises a hydroxycatecholate ligand. 5.The composition of claim 2, wherein the titanium complex contains twounsubstituted catecholate ligands and one substituted catecholateligand.
 6. The composition of claim 1, wherein the titanium complexcontains three unsubstituted catecholate ligands.
 7. The composition ofclaim 1, wherein the titanium complex further comprises at least oneligand selected from the group consisting of a substituted catecholate,ascorbate, citrate, glycolate, a polyol, gluconate, glycinate, ahydroxyalkanoate, acetate, formate, benzoate, malate, maleate,phthalate, sarcosinate, salicylate, oxalate, a urea, a polyamine, anaminophenolate, acetylacetonate, and lactate.
 8. The composition ofclaim 1, wherein the titanium complex has a formula ofM_(n)Ti(L1)(L2)(L3); wherein L1, L2 and L3 are ligands, and at least oneof L1, L2 and L3 is the at least one catecholate ligand; M is a mixtureof sodium and potassium counterions; and n is an integer ranging between1 and
 6. 9. The composition of claim 8, wherein substantially equalmolar quantities of sodium and potassium counterions are present.
 10. Anaqueous solution comprising the composition of claim
 1. 11. The aqueoussolution of claim 10, wherein the titanium complex has a concentrationof at least about 0.5 M in the aqueous solution.
 12. The aqueoussolution of claim 10, wherein the titanium complex contains at least oneunsubstituted catecholate ligand and at least one substitutedcatecholate ligand.
 13. The aqueous solution of claim 12, wherein the atleast one substituted catecholate ligand comprises a sulfonatedcatecholate ligand.
 14. The aqueous solution of claim 12, wherein the atleast one substituted catecholate ligand comprises a hydroxycatecholateligand.
 15. The aqueous solution of claim 12, wherein the titaniumcomplex contains two unsubstituted catecholate ligands and onesubstituted catecholate ligand.
 16. The aqueous solution of claim 10,wherein the titanium complex contains three unsubstituted catecholateligands.
 17. The aqueous solution of claim 10, wherein the titaniumcomplex further comprises at least one ligand selected from the groupconsisting of a substituted catecholate, ascorbate, citrate, glycolate,a polyol, gluconate, glycinate, a hydroxyalkanoate, acetate, formate,benzoate, malate, maleate, phthalate, sarcosinate, salicylate, oxalate,a urea, a polyamine, an aminophenolate, acetylacetonate, and lactate.18. The aqueous solution of claim 10, wherein the titanium complex has aformula ofM_(n)Ti(L1)(L2)(L3); wherein L1, L2 and L3 are ligands, and at least oneof L1, L2 and L3 is the at least one catecholate ligand; M is a mixtureof sodium and potassium counterions; and n is an integer ranging between1 and
 6. 19. The aqueous solution of claim 18, wherein substantiallyequal molar quantities of the sodium and potassium counterions arepresent.
 20. A flow battery comprising: a first half-cell containing afirst electrolyte solution; and a second half-cell containing a secondelectrolyte solution; wherein at least one of the first electrolytesolution and the second electrolyte solution comprises the aqueoussolution of claim 10.